Fix some XEmacs compilation issues.

* tex-buf.el (TeX-error-description-error): Make face definition
XEmacs compatible.
(TeX-error-description-tex-said): Ditto.
(TeX-error-description-help): Ditto.

* tex.el (nil): Handle the case of a non-available crm.el with a
`condition-case', instead of using the third argument of
`require', not recognized by XEmacs 21.4.
(and): Check whether dbus support is available before requiring
dbus.el.