(Folly/FBString) Use a lambda to throw an exception for NULL input

(Folly/FBString) Use a lambda to throw an exception for NULL input

Summary:
This should be standard compliant. `__PRETTY_FUNCTION__` seems
unnecessary, it just prints `basic_fbstring`, might as well use that and
simplify the code.

Test Plan:
`fbconfig --platform-all=gcc-4.8.1-glibc-2.17-fb folly/test`
`fbmake runtests_dbg`
`fbmake runtests_opt`

`fbconfig --platform-all=gcc-4.7.1-glibc-2.14.1 folly/test`
`fbmake runtests_dbg`
`fbmake runtests_opt`

`fbconfig --clang folly/test`
`...`

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1243670