Display warning when a "download_url" could not be generated from PURL in "Add Package"

[tdruez]

We recently added support for providing a PURL as the input of "Add Package" as an alternative to a download_url.
#131

When a PURL is provided, the purl2url is called to get a download_url but it cannot be done for all types of PURL.

The UI should be explicit about letting know the user that a download URL couldn't be generated and the consequences of it.

Related discussion:

• Guess/infer download_url optionally #152
• BUG: SBOM import does not trigger scan of packages #121
• DJC: Ensure that we can run a scan from a PURL in DejaCode #131