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. https://github.com/nexB/dejacode/issues/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:

• https://github.com/nexB/dejacode/issues/152
• https://github.com/nexB/dejacode/issues/121
• https://github.com/nexB/dejacode/issues/131