Change FBString's #define throw

Change FBString's #define throw

Summary:
Instead of defining throw to nothing, define it to something
which creates a grepable error message (re @tudorb in D1005889).

@override-unit-failures

Test Plan:
fbconfig -r folly
fbmake dbg && fbmake runtests && fbmake opt && fbmake runtests_opt

All tests passing.

Added throw to a few places in FBString; noticed that error messages
were produced, and that they contained the defined text. Those changes
not committed.

Reviewed By: tudorb@fb.com

FB internal diff: D1011963