glsl: Improve error message for interstage interface block mismatch.

We're now emitting this error from a point where we have easy access
to the name of the block that failed to match, so go ahead and include
that in the error message, as we do for intrastage interface block
mismatches.
glsl: Remove redundant writes to prog->LinkStatus

The linker_error() function sets prog->LinkStatus to false.  There's
no reason for the caller of linker_error() to also do so.