* Between 1.0.16 and 1.0.17 LOAD began to return NIL when the argument
had a non-NIL type and didn't name a file. Fixed with tests.
* Additionally, writing the tests exposed an odd type restriction:
LOAD's IF-DOES-NOT-EXIST argument was declared as having type
(MEMBER :ERROR :CREATE NIL), but the CLHS just says it's a