[MLIR] Fix confusing diagnostic during dialect conversion

Fix confusing diagnostic during partial dialect conversion. A failure to
legalize is not the same as an operation being illegal: for eg. an
operation neither explicity marked legal nor explicitly marked illegal
could have been generated and may have failed to legalize further. The
op isn't an illegal one per
https://mlir.llvm.org/docs/DialectConversion/#conversion-target
which is an op that is explicitly marked illegal.

Differential Revision: https://reviews.llvm.org/D116152