setup.py: improve error reporting when git commands fail. refs #2340

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
setup.py: use constant for _version.py filename.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
setup.py: set distribution.metadata.version from _version.py if we couldn't find it from git.
This simplifies reporting when we don't have the version. Also print the normalized version. refs #2340

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
setup.py: look for a tag named after APPNAME, not necessarily "allmydata-tahoe". refs #1146

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Merge pull request #125 from tahoe-lafs/2340-version-error-reporting-3

2340 version error reporting 3