MergeError is a ValueError

as suggested by @jreback

Author: thatneat

pandas/tools/merge.py

@@ -41,7 +41,7 @@ def merge(left, right, how='inner', on=None, left_on=None, right_on=None,
     merge.__doc__ = _merge_doc % '\nleft : DataFrame'
 
 
-class MergeError(Exception):
+class MergeError(ValueError):
     pass