[exec.general] Wording for AS-EXCEPT-PTR should use 'Precondition:' instead of 'Mandates:' for runtime properties

[lewissbaker]

The definition of the exposition-only AS-EXCEPT-PTR has a case for when the error is already an exception_ptr that has the following requirement:

• Mandates: err != exception_ptr() is true

"Mandates" clauses are usually for things which are ill-formed and can be detected at compile-time. However this is a runtime property and so should probably be a Precondition: instead.

Also, could be simplified to:

• Precondition: !err is false.