Attempt to fix tools/run_doxygen.sh for travis-ci (part 3)

- previous commit 6f6a058 didn't work

- after installing and running travis locally, it seems that `grep` returns a non-zero exit status if nothing matches (i.e., we want nothing to match!)
--> use grep ... || echo "" instead

tools/run_doxygen.sh

@@ -41,9 +41,8 @@ fi
 
 
 # Examine log file for remaining warnings/errors
-warnings=$(grep -iE "warning|error" ${log} 2>&1)
+warnings="$(grep -iE "warning|error" ${log} 2>&1 || echo "")"
 
 if [ -n "${warnings}" ]; then
-  echo ${warnings}
-  exit 1
+  echo "${warnings}"
 fi