MINOR: http-htx/proxy: Add http-error directive using http return syntax

MINOR: http-htx/proxy: Add http-error directive using http return syntax

The http-error directive can now be used instead of errorfile to define an error
message in a proxy section (including default sections). This directive uses the
same syntax that http return rules. The only real difference is the limitation
on status code that may be specified. Only status codes supported by errorfile
directives are supported for this new directive. Parsing of errorfile directive
remains independent from http-error parsing. But functionally, it may be
expressed in terms of http-errors :

  errorfile <status> <file> ==> http-errror status <status> errorfile <file>