BUGFIX: Capture exceptions when creating new user or updating password

[Benjamin-K]

This PR replaces #3901.

Upgrade instructions

Creating a user or changing the password will no longer throw an exception with a 500 error page but instead show the error in a flash message. This is done by wrapping UserService::addUser() and UserService::setUserPassword() with a try-catch block.

Review instructions

This PR changes the exception handling when creating new users or updating passwords by wrapping the used methods (UserService::addUser() and UserService::setUserPassword()) inside a try catch block and showing an error message (FlashMessage) if there was an exception.
That way it will be easier to extend Neos and add some password checks without showing a 500 error page in the Neos backend, which is currently the case with JvMTECH.NeosHardening (see jvm-tech/JvMTECH.NeosHardening#2) and maybe others.

With the adjusted code you can add checks to the password through an Aspect and simply throw an exception, if the requirements do not pass.

Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions

[Sebobo]

Besides the two tiny issues, looks fine by reading, thx!

[Sebobo]

Looks fine by reading

[paavo]

Any plans when this will be merged @Sebobo?

[kdambekalns]

Looks good by reading!