gitweb: Make .error style generic

gitweb: Make .error style generic

Style for td.error was introduced in 1f1ab5f (gitweb: style done with
stylesheet, 2006-06-20) to replace inline style for errors in old
multi-column "git annotate" based 'blame' view.  This view was then
since removed (replaced by "git-blame" based 'blame' view, with fewer
colums), making this style unused.

Make this style more generic by replacing td.error with .error to make
it apply to any element.  It will be used in 'blame_incremental' view
to show error messages.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>