olevba+mraptor: fix support of encrypted PowerPoint files

[decalage2]

In some cases, olevba and mraptor do not properly decrypt and analyse PowerPoint files (see #627):
it works fine with olevba on PPTM files (file is decrypted transparently, and the VBA macro code is properly extracted).
But it does not work well in other cases:

• For an encrypted PPT, olevba reports no macro (does msoffcrypto support PPT?)
• For encrypted PPTM and PPT, mraptor always reports "no macro".

The tools should decrypt files and report macros. And if they fail to decrypt, then they should report an error instead of "no macro".
Other tools should also be checked.