Specify message when query is blocked due to the default route behavior

[PavelLovygin]

…e behavior

[Denchick]

Hi there! Thank you for your contribution to our project! It seems like your PR fixes #803.

Could you please provide some tests for this PR? For instance, in the regress tests, you could explicitly set the default route behavior to BLOCK and check if you actually get an error with the specified message.

Smth like: CREATE TABLE denchick(i int) /* __spqr__default_route_behaviour: block */;