Merge: src/platforms: fallback to version field "0" on error when asking for a git_re...

Merge: src/platforms: fallback to version field "0" on error when asking for a git_revision

The _app.nit_ annotation `version` sets the version of mobile app packages (.apk and .app). For example, `version(1, 5, git_revision)` may produce the version string "1.5.6b42a7c".

This PR fixes an error when asking for a `git_revision` but the call to `git rev-parse` fails.
Note that the normal git error is printed before the nitc warning message, so debugging should be easy.

Close #2111

Pull-Request: #2113
Reviewed-by: Jean Privat <jean@pryen.org>