Add sanity check of local filenames.

* Some local filenames (filenames of downloaded files) contain just the version of a port,
  which does not make sense from a mirroring perspective. To protect against this,
  haikuporter now makes sure that the local filename contains the portname.
Improve error reporting a bit.

* Under some circumstances, haikuporter was less than helpful when recipes contained
  problems - one had to invoke it explicitly with -v to see what the actual problem
  was, sometimes even that didn't help. Always show these kind of errors now.
Fix/improve sanity check of local download filename.

* It's SRC_FILENAME, not SOURCE_FILENAME (thanks Humdinger!)
* Accept '-'/'_' mismatches.
* Accept lowercase/uppercase mismatches.
Drop sanity check on target filename again.

* The fallout is just too bad, sorry for the mess.