Feature Request: WCSAxes to display "BAD WCS" if image WCS is invalid

[pllim]

As suggested by @ejeschke :

https://github.com/ejeschke/ginga/pull/495#issuecomment-305682515

Maybe we should trap for WCS errors at the appropriate point in the canvas type and simply substitute a single text object (centered in the middle of the image) that says "BAD WCS" or some such.

https://github.com/ejeschke/ginga/pull/495#issuecomment-305832196

Well I am thinking of uses outside of the reference viewer, where there is/or may be, no info bar.