[libc++][NFC] Add error messages to a couple of static_asserts in span

Summary:
Add error messages to a couple of static_asserts in span to match the
style used in the rest of the file. Also fix an extra paren typo in a
assert error message.

Committed on behalf of Jason Lovett.

Reviewers: ldionne

Subscribers: libcxx-commits

Differential Revision: https://reviews.llvm.org/D52841

llvm-svn: 343725