More informative error for unparsable version

When Git is not installed, it is possible to get useless version strings
from versioneer, such as `0+unknown`.  Give a more informative error in
that case.

Author: matthew-brett

setup.py

@@ -55,7 +55,10 @@
 # 1.22.0 ... -> ISRELEASED == True, VERSION == 1.22.0
 # 1.22.0rc1 ... -> ISRELEASED == True, VERSION == 1.22.0
 ISRELEASED = re.search(r'(dev|\+)', FULLVERSION) is None
-MAJOR, MINOR, MICRO = re.match(r'(\d+)\.(\d+)\.(\d+)', FULLVERSION).groups()
+_V_MATCH = re.match(r'(\d+)\.(\d+)\.(\d+)', FULLVERSION)
+if _V_MATCH is None:
+    raise RuntimeError(f'Cannot parse version {FULLVERSION}')
+MAJOR, MINOR, MICRO = _V_MATCH.groups()
 VERSION = '{}.{}.{}'.format(MAJOR, MINOR, MICRO)
 
 # The first version not in the `Programming Language :: Python :: ...` classifiers above