Fix classnames to be BEM compliant

Change o-ft-forms-field-group to o-ft-forms_field-group
Change o-ft-forms-section__errorMsg to o-ft-forms-section__message--error
Cnange .o-ft-forms-globalMsg--error  to o-ft-forms__global-message--error
Update demo template to reflect change

Also includes an amend to border radius on form-section errors and global errors
Rather than using a literal as a fallback can you define a custom use case?