Fix formatting of errors for invalid vouchercode/regtype combination
Don't crash when using a validated-regtype + voucher

Validated-regtype is used for staff and speakers - if that is picked,
*and* a voucher is entered, an exception was thrown. This is clearly
never the right thing to do, but it should give an error and not crash.
Generate badges for confirmed registrations only

Normally we use the web interface which already did this, but if
using the one-off script, make sure we do the same there..
Allow specifying a badge foreground color for registration types

Requested after FOSDEM, since it's hard to read a black text on some
background colors.