#15 BBC micro and Thomson images are not recognised

[Bram Lohman]

The BBC Micro and Thomson image files are not recognised, and the EF does not complete the setup process to run these files

[Bram Lohman]

This was broken when checks for invalid files was added. The BBC Micro and Thomson images are not yet part of the "image type" enum and thus not recognised. Unrecognised images now throw an error and abort the emulation process.
BBC Micro and Thomson images should be added to the list of recognised images.

[Edo Noordermeer]

Implemented a temporary fix for this issue: Do not throw exception when disk image type is not recognised, but simply mount it. This works fine for BBC Micro and for Thomson images, but may be a problem later for unsupported file types (e.g. after adding a new Emulator).