Make sure the tooltip can be generated despite DevFail

[vallsv]

Here is a patch to allow to display tooltip for DevFail attributes.

Without catching this exception, the TaurusLabel devices can't start. As result.

This patch is not really nice cause the patched method have to return str (i am afraid about returning something else), then the formatting have to be done inside getDisplayDescrObj

This could be improved by:

• Returning the list containing value=exception, or an object which mimicks str
• The formatting could then be done inside TaurusBaseComponent.toolTipObjToStr
• The tooltip rendering could be then improved a lot

Closes #1086

[vallsv]

Here is an update.

[cpascual]

thanks a lot for the contribution @vallsv . Merged.