fix(common): don't crash on illegal unicodesets 🙀

- UnicodeSetItem was ignoring the error value from sizeUnicodeSet() and calling parseUnicodeSet with an invalid size
- it *is* an error, but should be handled properly.
- an error is already recorded in the callback

The symptom was an internal error where there should have been an error callback.

  2) vars
       should fail to compile: sections/vars/fail-uset-props1.xml:
     Error: Internal Error: uset returned error -1 for [:Adlm:]
      at new UnicodeSetItem
      at VarsCompiler.addUnicodeSet

For: #7377