[xbuild] / [PCL]: Report an error for unsupported / missing PCL Profiles.

We previously only reported a warning, then failed later with tons of
"The predefined type `System.Object' is not defined or imported", which
was very confusing to the user.

We now distinguish between missing Reference Assemblies (ie. none of
them are installed) and the requested profile not being available, reporting
a meaningful error message.