#2 Check return codes everywhere

[Markus Elfring]

Some checks for return codes are missing.

Examples:
Would you like to add more error handling for return values from "TIFFSetField" like in the function "tiffcp" and from "fprintf" in the function "usage"?
http://tifftools.cvs.sourceforge.net/viewvc/tifftools/tiffmesh/tiffmesh.c?revision=1.1&view=markup