[linker] Provide better error message when an error occurs while processing xml descr...

[linker] Provide better error message when an error occurs while processing xml descriptions.

With this mtouch will show:

> error MT2001: Could not link assemblies. Reason: Failed to process XML description: /work/maccore/master/maccore/tests/link sdk/extra-linker-defs.xml
>	Failed to process XML description: /work/maccore/master/maccore/tests/link sdk/extra-linker-defs.xml
>	Failed to resolve assembly: 'MonoTouch, Culture=neutral, PublicKeyToken=null'

instead of:

> error MT2002: Failed to resolve assembly: 'MonoTouch, Culture=neutral, PublicKeyToken=null'

which is a lot less helpful.