No checks for licence URI or typename

[goetzk]

licenses_uri_* and licenses_typename_* are required for minting to succeed but there is no sanity checking performed within the codebase to ensure they are present (however this fact is mentioned in the README).

                    my $licenseuri = $repo->phrase("licenses_uri_$license");
                    my $licensephrase = $repo->phrase("licenses_typename_$license");

These values should be checked during validation so entries can be fixed rather than having API calls to the DOI provider silently fail.