Add error handling for edge case of announcement with bad fname reference to widget

[apetro]

Summary

When a mascot announcement references a widget that doesn't actually exist, there's no error acknowledgement when the user tries to add the nonexistent widget to their home.

How to reproduce:

Given:

• have a mascot announcement that references a widget by an fname
• have that fname not actually map to anything (no portlet-definition with that fname

Then:

• access the mascot announcement
• click its inviting "Add to home" button
• observe mascot dismisses, with indication of failure
• inspect announcement again
• observe it now reports the widget as "On your home"

What should have happened:

• when the user attempts to add the widget to home, fail with a clear user-facing message
• do not incorrectly report widget as "On your home"

Credit where credit is due

Nonetheless: kudos on this already failing kind of gracefully. The page successfully renders, it doesn't fail out into the JavaScript console. It could fail gracelessly, rather than just silently. It's this graceful because developers are taking care.