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().
The pattern I'm following is from MergeError (declared on line 44). MergeError is raised when something is wrong with a caller's inputs to merge().

ValueError would also be an improvement but I figured I should follow the pattern used in the file.