Test connection using ping

Rework `DataSourceDefinition.prototype.testConnection` to invoke `ping` method in the workspace app.

Mark `DataSourceDefinition.testConnection` as deprecated, implement it using the original `testConnection` code running in the master process.

Improve error handling in `invokeMethodInWorkspace` to catch common errors and translate them to a useful error in the master process.

Some of the possible `error.code` values:
 - `ER_INVALID_CONNECTOR` is reported when the connector cannot be loaded, e.g. because it is not installed
 - `ECONNREFUSED` is reported when the juggler cannot connect to the database, e.g. because the host is not correct. This is probably MySQL-specific and may differ in other connectors.
 - `ER_ACCESS_DENIED_ERROR` is reported when MySQL credentials are not valid. A different error may be reported by other connectors.

See also https://github.com/strongloop/strong-studio/issues/109

/to @ritch please review