Improve error message for collimator errors

[Cédric Hernalsteens]

tctph: rcol, l=1.0, material="Cu";
tctph.4l2.b1: tctph, material="Cu", xsize=0.007393868206453028, ysize=10.0;

BDSCollimator::CheckParameters> half aperture bigger than width!
Full horizontal width is 600 mm for component named: "tctph.4l2.b1"
x (half) aperture 7.39387 mm, y (half) aperture 10000 mm

The error message could be clarified.

[Cédric Hernalsteens]

@Eliott Ramoisiaux Could you have a look at this?

[Laurie Nevay]

Fix Issue #304.

→ <<cset 27b02d3ac3ef>>