fix(custom-targets): fix bug in transactions, handle more failure cases, include more helpful text with failure responses

[andrewazores]

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---

Fixes: #487
See also cryostatio/cryostat-web#1262

Description of the change:

• fixes transaction handling bug (see fix(db): correct use of Transactional annotation #415)
• include better/more informative/more understandable error messages in responses
• check for duplicate URL or duplicate target (same JVM ID) cases in connection test, not only at creation time

How to manually test:

1. Check out and build PR
2. ./smoktest.bash -Otr
3. Go to Custom Target creation form
4. Try to create various custom targets like cryostat3:9091, localhost:0, sample-app-1:9093, jfr-datasource:11223, reports:11224

..

[aali309]

/build_test

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CI build and push: All tests pass ✅ (JDK17)
https://github.com/cryostatio/cryostat3/actions/runs/9322544977

[aali309]

Looks good to me.