More informative error for long MySQL username

We talked about it at https://github.com/serghey-rodin/vesta/issues/860 issue

func/main.sh

@@ -583,7 +583,10 @@ is_date_format_valid() {
 # Database user validator
 is_dbuser_format_valid() {
     exclude="[!|@|#|$|^|&|*|(|)|+|=|{|}|:|,|<|>|?|/|\|\"|'|;|%|\`| ]"
-    if [[ "$1" =~ $exclude ]] || [ 17 -le ${#1} ]; then
+    if [ 17 -le ${#1} ]; then
+		check_result $E_INVALID "mysql username can be up to 16 characters long"
+	fi
+    if [[ "$1" =~ $exclude ]]; then
         check_result $E_INVALID "invalid $2 format :: $1"
     fi
 }