glsl: Remove bogus check on return value of link_uniform_blocks().

A comment in link_intrastage_shaders(), and an if-test that followed
it, seemed to indicate that link_uniform_blocks() would return a
negative value in the event of an error.  But this is not the
case--all error checking has already been performed by
validate_intrastage_interface_blocks(), and link_uniform_blocks() can
only return unsigned values.

So get rid of the if-test and change the return type of
link_intrastage_shaders() to clarify that it can only return unsigned
values.
glsl: Add error message for intrastage interface block mismatch.

Previously we failed to link (which is correct), but we did not output
an error message, which could have been confusing for users.
glsl: Use a consistent technique for tracking link success/failure.

This patch changes link_shaders() so that it sets prog->LinkStatus to
true when it starts, and then relies on linker_error() to set it to
false if a link failure occurs.

Previously, link_shaders() would set prog->LinkStatus to true halfway
through its execution; as a result, linker functions that executed
during the first half of link_shaders() would have to do their own
success/failure tracking; if they didn't, then calling linker_error()
would add an error message to the log, but not cause the link to fail.
Since it wasn't always obvious from looking at a linker function
whether it was called before or after link_shaders() set
prog->LinkStatus to true, this carried a high risk of bugs.