The difference with FILE_ERROR is that this is for things that are
wrong with the path before looking at the disk.
Add some 3 tests; two broken as a reminder to actually use this new
code.
Apparently some systems actually have a directory called /nonexist[ae]nt.
It's hard to fathom a good reason for that, but oh well. As long as we
don't create such a directory inside the notmuch source tree, the new
version should be more robust.
This is not supposed to change any functionality from an end user
point of view. Note that it will eliminate some output to stderr. The
query debugging output is left as is; it doesn't really fit with the
current primitive logging model. The remaining "bad" fprintf will need
an internal API change.
