With the recent tightening of the code that is used to parse
various parts of a URL for use in the credential subsystem, a
hand-edited credential-store file causes the credential helper to
die, which is a bit too harsh to the users.  Demote the error
behaviour to just ignore and keep using well-formed lines instead.

* cb/credential-store-ignore-bogus-lines:
  credential-store: ignore bogus lines from store file
  credential-store: document the file format a bit more