Fix handling of dropping a property not associated with the given label

Fix handling of dropping a property not associated with the given label

When dropping a property by name from a label, the code checked only
whether the property existed in the graph's property catalog.  It did
not verify that the property was actually associated with the given
label, resulting in passing InvalidOid to performDeletion().  Fix it
by explicilty checking the label property association.

While at it also rearrange the code so as to avoid multiple ereport
calls for the same error in the same block.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/1DA5D52A-4AFA-426E-83F7-42ED974D682B%40gmail.com