Graceful error message for [Proof Mode] and [Set Default Proof Mode] when requiring a non-existing proof mode.
Graceful error message for [Proof Mode] and [Set Default Proof Mode] when requiring a non-existing proof mode.

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@14396 85f007b7-540e-0410-9357-904b9bb8a0f7

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