Error message is insufficient for blocked schema check approvals due to linting errors

[jdolle]

Linting errors prevent schema check approvals. This is the correct behavior, but the error returned to the client is generic.

Current error response is:

"error": {
"__typename": "ApproveFailedSchemaCheckError",
"message": "Schema check doesn't exist."
},

We should either:

1. Display a custom error when failing to approve a schema check due to a linting policy error
2. Modify the UI to disable schema approvals entirely for this check, with a clear explanation why.