Warn if the detected license isn't a valid SPDX, don't error

[befeleme]

When Copr adds a license-validate check to its repositories, pyp2spec doesn't have to be so strict in the automatic mode anymore: fedora-copr/copr#3290

Only warning should be sufficient for the human user, whom we expect to review the generated specfile anyway.

[trixirt]

The fedora-review will catch these problems so it is not necessary to error out.

[brianjmurrell]

Clearly something needs to happen here. Waiting for upstreams to fix metadata problems in their packages before COPR will successfully build them is just unworkable. I have been waiting literally weeks (if not months) for an upstream to simply fix their SPDX licence and it's still not done, making this package unbuildable in COPR.

[hroncok]

You don't need to use automatic pyp2spec conversion to build the package in copr.

[brianjmurrell]

I do if I want to use Anitya autorebuild on it and pyp2rpm fails to build the package.

Apart from the SPDX license barf, pyp2spec builds the package fine.