Not all "end" events have to be considered bogus connections

Instead, consider receiving a premature stream end while a query
is in progress as an error sign, bubbling up the error to the
client level. Fixes the testsuite while still fixing my cases
and possibly #458.

The correct fix would be to provide a .validate method to the
pooler, and expose a .validate method to the client and to the
connection classes for that purpose...