Make mutt.texi and mutt.info generation continue on failure.

Make mutt.texi and mutt.info generation continue on failure.

Add warning messages, noting the appropriate packages to install.

Note: mutt.texi won't normally be built unless configure finds one of
the possible expected conversion program names.  Unfortunately, the
canonical name, "docbook2texi", is used by both the docbook2x and
docbook-utils projects.

So on Arch, the canonical name is what we need, while on Debian and
Red Hat/Fedora it isn't.  We search for the alternative names first,
but it's still possible to invoke the wrong program if the needed
package is not installed.