Search for tags with the *.*.* patten for version

Description

The first patch in this set just gives a cleaner error message in the even that buildstream can not get a version number due to non numeric tag names.

The second patch changes to _version.py to search for the most recent tag that fits a regrex in this case ..*, this should filter out most text only tags but is not guaranteed, my regrex skills are not that hot.

The patch dose however change code that is auto generated by versioneer so we can wait and try and get this main lined in to versioneer or just merge and try and get the change mainlined in parallel.

Proposed changes

Changes proposed in this merge request:

• Catch this and similar errors and produce a cleaner message.
• Search for tags that are more likely to produce usable version numbers.

This merge request, when approved, will close issue/bug: #516 (closed)