Throw when prepare fails

[Pieter Hordijk]

AFAICS currently preparing a statement fails silently.

You will only find out it failed when trying to execute the statement resulting in the somewhat less useful message:

error Amp\Postgres\QueryError: ERROR: prepared statement "amp_46cfa66905e07b1e3b52558541a4bcf486056883" does not exist

Would be nice if errors on prepare would throw instead. So we have:

1. the error at the location it actually fails
2. have access to the actual error message.