typically non-zero exit codes are reserved for unexpected failures. doing a 
pull and not getting anything when no changes have been submitted isn't really 
unexpected.
It's easy enough to make a check to see if there was no submits anyway if a 
message is printed.

hg pull ... | grep "No new checkins"

for example.

Another way would be to actually print some sort of error code like apache 
does. Apache doesn't really have a failure itself if a document doesn't exist, 
but it prints a 404.
so maybe:

hg pull ... | grep "404: No new checkins"

which can obviously be reduced to
hg pull ... | grep 404

This may be good to use where error messages may change, but codes usually 
remain static.