Fix save_mysql_dbs for missing dbs

In change I075eb5a88113acfa36519e2c6e2aab87836be065 to workaround the
potentially missing dbs in a cells environment it turned off exiting on
error. This is a bad idea since it will mask real failures. This commmit
fixes the function to check if the db exists before trying to do a
mysqldump and report whether it's missing or not.

Change-Id: I54aac82e412a298cd1d2066a0fa51131a6da2af0

inc/upgrade

@@ -37,11 +37,16 @@ function save_mysql_dbs {
         set +o xtrace &&
             source $dir/stackrc &&
             echo ${DATABASE_USER:-root})
-    set +e
+    db_names=( $(mysql -u$database_user -p$mysql_pass -Bse "show databases;") )
     for db in $DATABASES_TO_SAVE; do
-        mysqldump -u$database_user -p$mysql_pass $db >$SAVE_DIR/$db.sql.$release
+        for db_found in "${db_names[@]}"; do
+            if [[ "$db" == "$db_found" ]]; then
+                echo "Saving database :" $db
+                mysqldump -u$database_user -p$mysql_pass $db >$SAVE_DIR/$db.sql.$release
+                break
+            fi
+        done
     done
-    set -e
 }
 
 # register a database we should save