Catch Non Numeric versions

This patch just displays a better message than the default stack trace
but dose not try to fix the problem. A further patch will be created but
it effects versioneer so may take longer to land as it may need to go
via versioneer mainline.

buildstream/utils.py

@@ -484,8 +484,13 @@ def get_bst_version():
         raise UtilError("Your git repository has no tags - BuildStream can't "
                         "determine its version. Please run `git fetch --tags`.")
 
-    return (int(versions[0]), int(versions[1]))
-
+    try:
+        return (int(versions[0]), int(versions[1]))
+    except Exception:
+        # We should try a mitigation hear, like falling back to a previous version.
+        raise UtilError("Cannot detect numeric Major and Minor version numbers\n"
+                        "Version: {} not in Interger.Interger.whatever format"
+                        .format(__version__))
 
 @contextmanager
 def save_file_atomic(filename, mode='w', *, buffering=-1, encoding=None,