Raise more specific exception on concat([None])

More specific exception allows for a safer, more explicit catch by a caller.
ConcatenationError follows the pattern of MergeError which is used in the same file when something is wrong with a caller's inputs to  merge().
Maybe this could just be a ValueError?

I'm not opposed to more specific exceptions, but if you are going to define a new exception, it probably should be exposed a higher level of the API than `pandas.tools.merge`. I'm guessing there is precedent for this somewhere?