Detail swapfile header inconsistencies.

Detail swapfile header inconsistencies.

Squid may fail to load cache entry metadata for several very different
reasons, including the following two relatively common ones:

* A cache_dir entry corruption.
* Huge cache_dir entry metadata that does not fit into the I/O buffer
  used for loading entry metadata.

Knowing the exact failure reason may help triage and guide development.
We refactored existing checks to distinguish various error cases,
including the two above. Refactoring also reduced code duplication.

These improvements also uncovered and fixed a null pointer dereference
inside ufsdump.cc (but ufsdump does not even build right now for reasons
unrelated to these changes).