No error thrown if megatools not installed

[benjibee]

Upon copy / paste of a link with or without decryption key and if megatools has not been installed, the app seems to function normally but does not initiate download.

That is to say, it may be a good idea to detect any external required tools and provide feedback when failing. The readme listed the first two steps as no. 1 (See PR #1) so I thought perhaps downloading the app was an alternative to installing megatools, which is obviously not the case!

[jacquesCedric]

Originally I planned to do a static compile of megatools with it's dependencies and bundle it with Ananke. However, I (unfortunately) learnt that macOS's gcc doesn't really allow for static c builds.

As a result the program is more of a GUI frontend to megadl, a component of megatools, than an alternative to it. I plan on implementing detection of required tools in a later build!

Cheers!