Check invalid styling and return a meaningful error

[ppazos]

Trying to add styling for bootstrap I set the value 'bootstrap' instead of 'bootstrap3' and I got an error: this.styles is undefined

It would be useful to actually check the values are valid and throw an error with the valid values to set on the styling option.

[ppazos]

same happens when setting bootstrap4 and bt4 was not included in the page, I get this error