Expand error messages for invalid license information

[ehuss]

This expands the error message when the license expression is not valid. It provides some help text if the user needs a custom license, and also points to the cargo documentation. It also includes the actual error message from the validator, which can be helpful if you have a more complex expression, and it may not be clear exactly which part of the expression is invalid.

This also clarifies the wording when license is not specified.

The error may look something like this:

error: failed to publish to registry at http://localhost:8888/

Caused by:
  the remote server responded with an error: unknown or invalid license expression; see http://opensource.org/licenses for options, and http://spdx.org/licenses/ for their identifiers
  Note: If you have a non-standard license that is not listed by SPDX, use the license-file field to specify the path to a file containing the text of the license.
  See https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields for more information.
  MIT AND foobar
          ^^^^^^ unknown term

Closes rust-lang/cargo#3540

[Turbo87]

since we are including the original SPDX error now, could you add a test case for this? :)