Exception handling for special characters in attribute value

[sbanoeon]

Describe the bug
There is unclear message when including forbidden special characters in attribute value and posting entity. Currently it throws an error "Invalid characters in attribute value"

Expected behavior
A clear message indicating which forbidden character is part of the submitted request.

[djs0109]

There are validators in utils.validators.py. But it seems like this is not being applied for attribute value, which can also cause a problem

[github-actions]

Branch 330-Exception-handling-for-special-characters-in-attribute-value created!