Don't crash on unexpected license value in Project.toml

[ericphanson]

          I guess we expected packages to do `license = "BSD-2-Clause"` instead of

[license]
SPDX = "BSD-2-Clause"

Either way, we shouldn't crash on this but rather emit a debug message and mark the license in project as missing. We could also try to add support for this format.

Originally posted by @ericphanson in #103 (comment)