When constant-folding 'gettextureinfo', upon failure, don't suppress the...

When constant-folding 'gettextureinfo', upon failure, don't suppress the
error message and assume it will get re-issued at shade time; instead,
constant fold the gettextureinfo op into a call to error()!

For those following along, this is really part 2 of:
https://github.com/OpenImageIO/oiio/pull/504
If you turn on the "one_error_per_file" option in OIIO, you need this.