BibDocFile: handling of exceptions in Md5Folder

* Improves handling of exceptions in Md5Folder by not causing automatic
  reload of md5 in case of Exceptions. (closes #1060)

* Additionally, always use register_exception(alert_admin=True) in case
  of MD5 related issues, as these are severe enough to call for further
  human check.

* Introduces Md5Folder regression tests.

Reported-by: Alessio Deiana <alessio.deiana@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>