If there is a crash landing, then the Planetary Protection Officer (ESA's in the case of Schiaparelli) documents the crash location and the organic and biological material that was likely dispersed. This is available to all of the agency Planetary Protection Officers in order to provide information and advice for future missions to consider how that contamination could impact their experiments.
The mission needs to provide an assessment before launch of the probability of a crash landing. However for Category IVa there is no requirement on that probability, and it is most certainly not expected to be zero. Instead the mission is required to provide before launch an inventory of the organic materials in the spacecraft, so that if it does crash, the degree of contamination can be estimated for future missions to Mars. You would expect that a future life or organic material detection mission would not go near a previous crash site.
Category IVa missions must clean the outside surfaces of the spacecraft to constrain the biological burden that Mars would be exposed to on a normal landing. However there are many parts inside the spacecraft that are not so cleaned because they are not expected to be exposed, unless the landing fails dramatically. A simple example is the Inertial Measurement Unit, which is a sealed device that was built with no biological cleanliness requirements. If that thing breaks open on landing, then there will be more biological contamination of the impact site than for a normal landing.
There is no evidence that any of the crashes have brought components into contact with subsurface water. IVa missions cannot target environments that are considered likely to have near-surface subsurface water. These are called "special regions".
From the Planetary Protection Provisions for Robotic Extraterrestrial Missions:
Special Regions are defined as areas or volumes within which
sufficient water activity AND sufficiently warm temperatures to permit
replication of Earth organisms may exist.
Missions to such locations are deemed Category IVc (whether or not they are looking for evidence of life), which require much more stringent sterilization. The surface bioburden requirement is four orders of magnitude lower than for IVa. If the probability of a crash landing is assessed to be more than 1%, then the innards have a bioburden requirement as well, though not as stringent. (At this point, I would consider it very unlikely that any project planning to land something on Mars would be able to successfully defend a claim that their probability of a crash landing is 1% or less.)
The only Mars missions' landers that has been subject to such sterilization to date is Viking, which subjected the entire lander systems to a dry heat sterilization process and encapsulated the whole thing to prevent recontamination before launch. Some parts of Beagle 2 were sterilized, but not the whole thing. Some parts of Mars 2020 will be sterilized and encapsulated, mainly the parts that will be in contact with collected samples.
None of the requirements are zero. There can and likely will be some potentially viable bacterial spores on the surface of lander targeted even to a special region, and many inside.
From the reference, the project is required to document at end of mission, for any category, whether the landing was successful or not:
(6) An inventory of bulk constituent organics that includes:
(a) Parts lists, material lists, and other program documentation containing data relevant to organic material identification that are prepared by a flight project to specify and control the materials that are included in a vehicle destined for planetary landing.
(b) The locations of landings and impact points (determined and defined as accurately as mission constraints permit) of major components of space vehicles on the planet surface,
(c) Estimates of the condition of each landed spacecraft to assist in calculating the spread of organic materials.
If the lander crashes, The PPO documents the crash location along with an estimate of the biological contamination of the site.
Here are the specific IVa requirements:
18.104.22.168 Category IVa. Lander systems not carrying instruments for the investigations of extant martian life shall be restricted to a surface biological burden level of ≤ 3 x 105 spores, and an average of ≤ 300 spores per square meter.
a. An assessment of the probability of a non-nominal landing (including EDL) shall be provided.
The keys there are "surface" and "non-nominal landing". The non-nominal landing is NASA terminology for a crash landing. Only the surfaces need to be cleaned for IVa, and the project needs to tell the Planetary Protection officer what they think the probability of a crash landing is.