#144 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 "pthread_mutex_init" like in the constructor for "PthreadMutex" and from "fwrite" in the member function "ConfigFile::Save"?
http://avifile.cvs.sourceforge.net/viewvc/avifile/avifile-0.6/lib/common/Locker.cpp?revision=1.10&view=markup
http://avifile.cvs.sourceforge.net/viewvc/avifile/avifile-0.6/lib/common/Config.cpp?revision=1.12&view=markup