Support for multiple errors to MapScript

[Tamas Szekeres]

I have just added a support for multiple error messages to be dispatched to
SWIG mapscript according to bug 1704. This change will prevent the older
messages being hidden by the newer ones within a call ("failed to draw layer
.." was not too descriptive).

The languages not using the common %exception in maperror.i will not benefit
from this change (except C#) so those bindings might have to be updated
accordingly (by calling msGetErrorString for example).


Tamas