mesa: fix proxy texture error handling in glTexStorage()

mesa: fix proxy texture error handling in glTexStorage()

This is basically a follow-on to 1f5b1f98468d5e80be39e619ed15c422fbede8d3.
Basically, generate GL errors for ordinary invalid parameters for proxy
targets the same as for non-proxy targets.  Only texture size and OOM
errors should be handled specially for proxies.

Note: This is a candidate for the stable branches.