Add ability to attach CPLErrorHandler via C# swig bindings

[project10]

I would like to have the ability to register my own CPLErrorHandler via the C# SWIG bindings. Presumably this would involve passing in an appropriate delegate to a static Gdalconst method, which would in turn invoke CPLSetErrorHandler behind the scenes.

This would be used to read any log messages generated internally to GDAL/OGR - including any at debug levels, assuming that CPL_DEBUG is set appropriately.

I am unsure how this would play with "UseExceptions()", I assume that setting a custom error handler would override the error handler that UseExceptions uses to turn error messages into exceptions.

[tamas]

Added this support in r18358 see the usage example attached