Fix panic when blame-palette is empty

[thorio]

Currently, when blame-palette is set to an empty string, the application panics:

$ git blame ARCHITECTURE.md | delta --blame-palette ""
thread 'main' panicked at src/handlers/blame.rs:175:47:
attempt to calculate the remainder with a divisor of zero
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This patch replaces this with a nicer error message:

$ git blame ARCHITECTURE.md | delta --blame-palette ""
Option 'blame-palette' must not be empty.

[dandavison]

Thanks @thorio! I'm thinking that this should go earlier since it is essentially user input validation. Perhaps in src/config.rs where we first parse the argument. (Or maybe clap has a "value parsing" feature that we can use?)

[thorio]

I thought about that, but since the actual color is only parsed after this point, I just stuck the check here for simplicity.

Ideally clap would parse the color and palette, then store it in a struct or enum guaranteeing its validity, but that would be a larger change.

As for just moving it earlier in the execution, I think right after this line would be a good spot.

[dandavison]

Thanks!