improve message for authentication errors

[anuragsoni]

During the startup message flow postgres will shutdown the socket immediately after sending an error frame. Trying to read further from the socket will always result in EOF. If we instead exit early by raising the exception with Error_response, the error message seen by pgx users will be a little better. For a login failure, before this PR we would see something like:

(monitor.ml.Error (Pgx_async.Pgx_eof)
 ("Raised at Pgx_async.Thread.input_char.(fun) in file \"pgx_async/src/pgx_async.ml\", line 50, characters 14-27"
  "Called from Async_kernel__Deferred1.M.map.(fun) in file \"src/deferred1.ml\", line 17, characters 40-45"
  "Called from Async_kernel__Job_queue.run_jobs in file \"src/job_queue.ml\", line 167, characters 6-47"))

With the diff in this PR the error will show up as:

(monitor.ml.Error
 (pgx/src/types.ml.PostgreSQL_Error
  "Failed to authenticate with postgres server"
  ((code 28000) (severity FATAL)
   (message "Peer authentication failed for user \"faa\"")
   (custom ((R auth_failed) (L 330) (F auth.c) (V FATAL)))))
 ("Raised at Pgx.Make.connect.(fun).loop.(fun) in file \"pgx/src/pgx.ml\", line 645, characters 8-85"
  "Called from Async_kernel__Deferred0.bind.(fun) in file \"src/deferred0.ml\", line 54, characters 64-69"
  "Called from Async_kernel__Job_queue.run_jobs in file \"src/job_queue.ml\", line 167, characters 6-47"))

[coveralls]

Coverage remained the same at 66.848% when pulling 2917b46 on improve-error-reporting into e408363 on master.