Don't discard errors during snapshot/asset loading

[MariusDoe]

This leads to confusion because the error is treated as if the code/asset was not present, leading to apparent deletions in the change sets.

Instead, the error should simply not be caught and a file/directory exists check should be added before loading. This is enough, as mappers should be provided a commit in which they are tracked (which should imply that the code/asset is present) or an empty commit (which cannot contain any other mapped file).