Use ereport(ERROR) instead of Assert() to emit syncrep_parser error.

Use ereport(ERROR) instead of Assert() to emit syncrep_parser error.

The existing code would either Assert or generate an invalid
SyncRepConfig variable, neither of which is desirable. A regular
error should be thrown instead.

This commit silences compiler warning in non assertion-enabled builds.

Per report from Jeff Janes.
Suggested fix by Tom Lane.