Fail gracefully if disable_warnings not available
Fail gracefully if disable_warnings not available

Addresses #418. Rationale explained there.

PKRoma/httpie,jkbrzt/httpie,jkbrzt/httpie,jakubroztocil/httpie,jkbrzt/httpie,jakubroztocil/httpie,PKRoma/httpie,jakubroztocil/httpie