1.0.17.26: Fix regression in LOAD

1.0.17.26: Fix regression in LOAD

* 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
  generalized boolean.