Improve error message quality of exceptions

[jng]

There are a series of exceptions thrown in MapGuide that could be easily improved in terms of clarity. For example:

• MgNullArgumentException - Should include the identifier that failed the null pointer check
• MgNullReferenceException - Should include the identifier that failed the null reference check
• MgObjectNotFoundException - Should include the name whose associated object could not be found

Most of these exceptions are thrown by pre-defined macros like CHECKARGUMENTNULL(), etc. So a simple modification to these macros where applicable to include the stringified argument being checked will result in clearer error messages being included.