Numerous small form enhancements to support JENKINS-38597:

• Prevent radio button from shrinking in constrained layout (check checkbox / favorite too)
• Only show the FormElement header's divider when it's a direct child of a FormElement with error state (to support nesting of FormElements)
• Apply some default spacing for nested FormElements
• Simple "error message" component for errors not tied to a specific FormElement
• Fix styling on PasswordInput