Fix problem with arcs with negative radius

[breiler]

Fixes a problem where a program like this could not be loaded:

G17 G21 G90 G94 G54 M0 M5 M9
G0 X10  Y10.0   F8.0
G02 X20 Y0      R-10

NCViewer displays it like this:

There were two problems. A check was in place to prevent negative radius values which was removed. But we also needed to handle the special case to invert the angle when there is a negative radius. Of there these fixes it now displays correctly with:

Also prevent spamming error dialogs when loading gcode with multiple errors. Removed unused parameter for spindle speed.

Relates to #2710