#42 Warn user on invalid matrix size request

[Mike Laughton]

dmtxwrite should throw a more descriptive warning when a user requests an invalid matrix size:

Current behavior:
$ echo -n 123456 | dmtxwrite -p -s10000x10000
Usage: dmtxwrite [OPTION]... [FILE]
Try `dmtxwrite --help' for more information.

Improved behavior:
$ echo -n 123456 | dmtxwrite -p -s10000x10000
dmtxwrite: Invalid matrix size: "10000x10000"
Usage: dmtxwrite [OPTION]... [FILE]

(filed on behalf of Jernej)
Try `dmtxwrite --help' for more information.