sanitycheck: advice about possible docbook pkg name

In the same way autogen.sh advices about possible name for the
'gnome-common' package, do the same in sanitycheck command when
not finding Docbook components in the XML catalog.

https://bugzilla.gnome.org/show_bug.cgi?id=720486
sanitycheck: extract try block into a function

This is more readable and will be useful for the fix in the
subsequent commit.

https://bugzilla.gnome.org/show_bug.cgi?id=720486
sanitycheck: don't check M4 if autoconf is not present (bgo#720486)

If autoconf is not installed, get_aclocal_path() would throw, and
its exception would simply be printed without any indication of
the problem. It's simply better to not check for M4 macros if
autoconf check already failed before.

https://bugzilla.gnome.org/show_bug.cgi?id=720486