Add a function `ForceConsoleColor`, like `DisableConsoleColor` but to force coloring the output.
It usefull when some IDE's integrated console (like IntelliJ or Goland) are not detected as TTY, but can display colors.
Also helps if one want to output color in log file (#1590) and as a workaround for #1547.
1. Why is this change neccesary?
White text on a yellow background was illegible with most terminal color schemes
2. How does it address the issue?
The white text was replaced with a bash compatible dark gray while keeping the
yellow background colour
3. What side effects does this change have?
Resolves#1552
Otherwise, caller needs to invoke WriteHeaderNow himself after
AbortWithStatus(), which is error-prone.
Also modified ErrorLoggerT() such that it always writes log to response
body. Otherwise calling AbortWithStatus() will fail to write body because
c.Writer.Written() is set true by WriteHeaderNow().
