MINOR: http-htx: Add understandable errors for the errorfiles parsing

MINOR: http-htx: Add understandable errors for the errorfiles parsing

No details are provided when an error occurs during the parsing of an errorfile,
Thus it is a bit hard to diagnose where the problem is. Now, when it happens, an
understandable error message is reported.

This patch is not a bug fix in itself. But it will be required to change an
fatal error into a warning in last stable releases. Thus it must be backported
as far as 2.0.

(cherry picked from commit a66adf41ea28a0fa29437d1675f225b5cc589b59)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 3a10710b777370ef36763b8b6658ae63ef5c4ec4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 58f55acf4eafd19ed35ad8506064198229ba69c3)
[cf: Adapted for the 2.1]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>