Print a more meaningful error message when help can't run man.
Print a more meaningful error message when help can't run man.

Signed-off-by: Zoran Zaric <d7dacae2c968388960bf8970080a980ed5c5dcb7@zoranzaric.de>
[1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org: elaborated on the error message a bit]
Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

pombredanne/bup,pombredanne/bup,jbaber/bup,gevaerts/bup,ToxicFrog/bup,apenwarr/bup,gevaerts/bup,apenwarr/bup,jbaber/bup,pombredanne/bup,ToxicFrog/bup,mhoeher/bup,tjanez/bup,tjanez/bup,apenwarr/bup,gevaerts/bup,apenwarr/bup,jbaber/bup,tjanez/bup,gevaerts/bup,gevaerts/bup,mhoeher/bup,mhoeher/bup,pombredanne/bup,jbaber/bup,ToxicFrog/bup,mhoeher/bup,jbaber/bup,ToxicFrog/bup,pombredanne/bup,mhoeher/bup,tjanez/bup,mhoeher/bup,tjanez/bup,tjanez/bup,ToxicFrog/bup,jbaber/bup,pombredanne/bup