Check the return value of RT::URI->FromURI instead of ->Resolver

FromURI always sets a resolver, even on failure.  Error checking which
only looks at ->Resolver is broken and checking ->Scheme on top of that
is just patching over the bug and relying on the internals of
RT::URI::base.  FromURI returns a useful success or failure value; just
use that.
Return a (status, message) tuple on success from RT::Link->LoadByParams

Previously only returned a tuple on failure, which is less than useful.
Merge branch '4.0/check-FromURI-return-value' into 4.0-trunk