shadertool.py: Match x.ye+z floats

Already matched e- notation, just forgot about e+ :-p
shadertool.py: Add --ignore-parse-errors option

Useful if bulk installing all shaders to skip any that can't be parsed
(I hit this on a ps_1_1 with a random + before a mov)
shadertool.py: Don't barf if a pixel shader doesn't take any texcoords

Came across one that copies a constant input to it's output without
using any texcoords.