Fix #10804 Prevent creation of duplicate tags

[allyoucanmap]

Description

This PR adds a new error message for tag name already existing.

Important: PR geosolutions-it/geostore#416 backend side needs to be merged

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/geosolutions-it/MapStore2/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x", remove the others)

• Bugfix

Issue

What is the current behavior?

#10804

What is the new behavior?

A new message error when a tag name already exists

Breaking change

Does this PR introduce a breaking change? (check one with "x", remove the other)

• No

Other useful information

[offtherailz]

Instead of saying "A tag with the same name already exists" the proper message is "the tag <...> already exists". or "The tag you are trying to create already exists"