parse server error messages (#543)

This commit adds a parser to the Postgres error message, providing better
error messages.

Implemented based in:
  https://www.postgresql.org/docs/12/protocol-error-fields.html

Signed-off-by: Sebastian Webber <sebastian@swebber.me>

src/errors.rs

@@ -12,6 +12,7 @@ pub enum Error {
     ProtocolSyncError(String),
     BadQuery(String),
     ServerError,
+    ServerMessageParserError(String),
     ServerStartupError(String, ServerIdentifier),
     ServerAuthError(String, ServerIdentifier),
     BadConfig,