BUG: fftconvolve should raise error for invalid mode instead of doing nothing
BUG: don't use _valfrommode for correlate/convolve, since it accepts a numerical mode without raising an error, but then correlate doesn't handle it correctly.
Merge pull request #3090 from endolith/fftconvolve_mode_error

BUG: signal: fftconvolve should raise error for invalid mode instead of doing nothing

Ditto for mode handling in correlate()