Fix internal return value that prevented error handling. (#204)

In case of early exit without retries from function copydb_copy_table we
would break out of the retry loop and return true, allowing pgcopydb to
continue working and report a successful return code to the OS.

In passing, also review error handling in lower-level pg_copy function to
prevent trying to COMMIT a transaction in an already failed connection.

Finally, we could have either a client-side connection error or a
server-side reported connection error: improve our code to take both the
situations into account.