pytype: only output the "pytype crashed" message on error

If pytype did not crash while generating stub, that message is kind of
confusing. It seems simple enough to avoid it in this case.

contrib/check-pytype.sh

@@ -127,5 +127,7 b' pytype -V 3.7 --keep-going --jobs auto \\'
     -x mercurial/wireprotov1peer.py \
     -x mercurial/wireprotov1server.py
 
-echo 'pytype crashed while generating the following type stubs:'
-find .pytype/pyi -name '*.pyi' | xargs grep -l '# Caught error' | sort
+if find .pytype/pyi -name '*.pyi' | xargs grep -ql '# Caught error'; then
+    echo 'pytype crashed while generating the following type stubs:'
+    find .pytype/pyi -name '*.pyi' | xargs grep -l '# Caught error' | sort
+fi