Fix to the rest of the Guitar Pro import faults

Updates to the loading to GP3 and GP4 formats. The files I have now are all fine, any failing in the archives are either:

1. Actually HTML files with a GP extension;
2. Do not identify as Guitar Pro files in their header.

For 1, we simply reject them with Bad Format as we should. (Perhaps for 2 we should at least try to load the file based on the extension rather than the header, but as they are not created in Guitar Pro they could possibly fail later anyway.)