MSGIDs added to warnings and errors

MSGIDs allow for better error checking.
Also used the built-in sprintf functionality of error/warning
where string concatenation was used.
Replaced errors starting with "Function XXX" with a proper
MSGID of the form  "matlab2tikz:XXX:msg"
Merge pull request #238 from egeerardyn/msgid

MSGIDs added to warnings and errors