You can try the following nested fields (of course, "test" should be
replaced with *your* bookmark name):

{ IF { REF test } = "Error! Reference source not found." "" "{ REF
test }" }

Press Ctrl+F9 to insert each pair of field delimiters, { }. Press F9 to
update. Use Alt+F9 to show/hide field codes.

Note that this assumes that "Error!..." is the exact error message that
is occurring. Of course, if you are sending documents to people using a
different language version of Word, the error message will also be
different, and the code won't work. To get around this, you would have
to get more creative. Peter Jamieson has suggested this approach:

{ SET errmsg { REF NonExistingBookmark } }

{ IF { REF test } = { REF errmsg } "" "{ REF test }" }

Here, "test" should be replaced with your bookmark name, but
NonExistingBookmark should be used (the point is that it should be
non-existing and therefore generate the error message).

For more, see the following thread:

Checking to see if a field reference or bookmark exists
http://groups.google.se/group/microsoft.public.word.docmanagement/browse_thread/
thread/17b44a4a15ef8de4/