Pass Attribute to _get_read_widget instead of DataType

[GDYendell]

This will allow better error reporting when exceptions are raised. Update EPICS CA exception for 2D waveforms to include the attribute.

[jacob720]

Should the same be done for _get_write_widget?

[GDYendell]

Should the same be done for _get_write_widget?

Good point. It doesn't need it currently, but yeah I think it should be consistent.