hghave: make error output easier to diagnose

I had a typo that meant the new bash check was throwing an exception
(due to a missing argument), but it was very hard to diagnose without
this change.

Differential Revision: https://phab.mercurial-scm.org/D10733

tests/hghave.py

@@ -104,8 +104,8 b' def checkfeatures(features):'
         check, desc = checks[feature]
         try:
             available = check()
-        except Exception:
-            result['error'].append('hghave check failed: %s' % feature)
+        except Exception as e:
+            result['error'].append('hghave check %s failed: %r' % (feature, e))
             continue
 
         if not negate and not available: