There are default error messages such as "Please fill out this field", if the field is set as required but the user leaves it blank. Is there a way to customise these messages?
That message is generated from the browser when the word required is on an input (that supports it), it's not actually part of Bootstrap. I believe that it is an HTML5 thing though.
To do what you want, you would have to create for yourself a FORM validation script in which you can then display whatever message you want for errors etc..
I'm sure that you can search for a JS plugin to use, there a quite a few to choose from. Since JQuery is already included and if your familiar with it, I'd suggest looking for a plugin based on using JQuery.