Improve incorrect or unspecific output for "autoconf"

Some plugins emit wrongly formatted "no" messages or lack the "yes"
message on success.

plugins/mysql/mysql_report

@@ -60,13 +60,12 @@ default_args="--base 1000 -l 0"
 default_scale="no"
 
 if [ "${1}" = "autoconf" ]; then
-  result=0
   if [ -z "${mysqlbin}" ]; then
-    echo "no"
+    echo "no (missing mysql executable)"
   else
     echo "yes"
   fi
-  exit $result
+  exit 0
 fi
 
 if [ -z "${names}" ]; then