The proxy backend form is closed when proxy path isn't unique

[marla-singer]

Verified on nightly.apinf.io, multi-proxy case

Steps:

1.Create an api
2. Go to Proxy tab and add the next values:

• Proxy: Nightly umbrella
• Proxy base path: /new-proxy-path/
• Api base path: /

3. Click on button Save & Publish

Found:

• The form is closing and user can't understand why

Reason:

• The proxy base path isn't unique

Expected:

• The form is opened and an error message is shown

[jykae]

Assigned by @bajiat