Exit gracefully when trying to build in a directory without configuration file.

[Bzero]

A small PR to ensure a graceful exit and provide some feedback on why building failed when trying to do it in a directory missing a configuration file.

[lawmurray]

Thanks for this one @Bzero. Let me review.

I recall wanting, in the past, to allow doxide to run without a configuration file for trivial cases, but this might be misguided now as features have expanded, and this seems an opportune time to revisit.

[Bzero]

I see, in this case feel free to reject this PR if you decide to fall back to some default when there is no configuration file.

[lawmurray]

Let me merge this one too @Bzero, better to err on the side of not creating files when run in the wrong directory I think. I extended the actual error message displayed to suggest an action.