Make ErrorMsg color distinct from ServerMsg

Switch ErrorMsg to a reddish color rather than the golden hues of
ServerMsg, matching the behavior of Quasseldroid.

This helps with discovering if Auto Identify fails now that SASL
authentication errors are reported in the server buffer.  It also
points out any messages Quassel doesn't handle correctly, which might
be unexpected, but should help with discovering them.

No settings migration logic included as custom colors are an
all-or-nothing approach.  Due to 'default.qss' using a system
palette color, there's no easy way to upgrade without changing
existing colors or loading all of Qt's color palettes in startup.
For example, palette(text) is dependent upon the system theme:
ChatLine {
  foreground: palette(text);
}