Fix bogus switch cases in declgen.

One of the switch statements in declgen has completely bogus case
labels: '0', '1', ... instead of 0, 1, ...

Fix this up and also add a default: assert(0); which would have caught
this earlier.  Improve the test case output when a randomdecl failure
causes the test to fail.

Also add a comment to the one switch statement in this file which
intentionally does not have a default: assert(0).