Update $error-colour variable to use existing colour palette
Update $error-colour variable to use existing colour palette

This change updates the $error-colour variable from #af1324 to the
existing $red colour variable #b10e1e, rather than introducing another,
similar shade of red.

#af1324 has a contrast ratio of 7:14:1, however, the palette already
has $red: #b10e1e; which has a contrast ratio of 7:12:1.

This colour should provide enough contrast between text and its
background so that it can be read by people with moderately low vision.

quis/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit