Improve error message and merge heuristic

[hyperupcall]

Hello again!

I came across two bugs in code-owner-self-merge:

• Inaccurate error message, when user tries to merge PR with "LGTM", but the mergability check has not been completed. This code path was seen in https://github.com/SchemaStore/schemastore/pull/3795 (more details in commit message)
• When "LGTM" is in quotation marks, it should not merge since that is not the intent (imo). This bug was seen activated in https://github.com/SchemaStore/schemastore/pull/3824

This PR fixes those errors. These changes have been active at SchemaStore/schemastore for a week, and things seem to work as expected.

Related to 8714b4990278162db0fb87717dd4a1e56fbd7a36 and #34.

[orta]

👍🏻 this all seems very reasonable to me, nice work