Improve error reporting, include MySQL URI and socket error codes in all connection errors

[clue]

This changeset improves error reporting to include the MySQL URI and socket error codes in all connection errors:

- Error: Unable to connect to database server
- Error: Access denied for user 'test'@'::1' (using password: YES)
+ Error: Connection to mysql://test:***@localhost/test failed: Connection to localhost:3306 failed: Last error for IPv4: Connection to tcp://127.0.0.1:3306?hostname=localhost failed: Connection refused. Previous error for IPv6: Connection to tcp://[::1]:3306?hostname=localhost failed: Connection refused
+ Error: Connection to mysql://test:***@localhost/test failed during authentication: Access denied for user 'test'@'::1' (using password: YES) (EACCES)

Builds on top of #138, clue/reactphp-redis#116, clue/reactphp-http-proxy#17 and others