Fix gettext download error and improve download failure exception usability

commit 25b23412:

cerbero/source: Use ftp.gnu.org instead of ftpmirror
ftpmirror points to random GNU mirrors which are community maintained
and are likely to be down.

commit a6a5738c:

cerbero/source: Make download failure exceptions usable
Now we print the original URL that failed to download, then a tuple of
murl, exception so users can figure out what happened with the
mirrors.

commit 5d1318c4:

cerbero: Capture stderr to logfile for git.get_hash()

commit d150794e:

cerbero/git: get_hash() is allowed to fail
The fetch command relies on this to be allowed to fail, since it could
be called for a git recipe that has never been built.

commit efc4a086:

cerbero: Ensure that get_hash() is always quiet
If the logfile is not present (such as when running the fetch cerbero
command), then we still don't want to see stderr since it is often run
on repositories that have nothing in them when a recipe has never been
built before.