[PATCH] Clean up property graph error messages

Hi,

While looking at the SQL/PGQ property graph error paths, I noticed a
few small cleanups in propgraphcmds.c.

The attached patch fixes a user-visible error message from "mismatching
properties names" to "mismatching property names", and moves a
ReleaseSysCache() call before an ERROR ereport in
check_element_properties().

The existing code should be cleaned up by
the resource owner on the ERROR path, but the explicit ReleaseSysCache()
placed after ereport(ERROR) was unreachable.

The regression expected output is updated for the changed error text.

[On a separate note, it might be better to change all "vertexes" to
"vertices",
haven't included that in the patch though since the other one is not exactly
wrong?]

Regards,
Ayus