Use %s instead of %g in findroot error
Use %s instead of %g in findroot error

This fixes incorrect cutoff for tolerances that don't fit in a Python float.

This causes the error message to be longer, especially when the precision is
set high. Perhaps there is a better way to only print a few digits without
using %g.

Fixes an issue mentioned in #339.

JensGrabner/mpmath,fredrik-johansson/mpmath,JensGrabner/mpmath