fix: Do not panic on multiple install calls

[jaysonsantos]

When writing tests it is really useful to use color-eyre to have better messages but, calling install multiple times will panic.
This should return an error instead.
It should fix #97

[jaysonsantos]

I wonder if it would be better to use sync::Once on color_eyre::install to make sure only one instance is being installed using a singleton, what do you think? let me know and i can change the pr

[yaahc]

I think I prefer this version as is over doing it in a Once. It's easy enough¹ to write a wrapping function using a Once if you're going to intentionally race a bunch of installs, and in my experience color-eyre isn't the only library where you need to worry about initializing it once in tests so leaving that to the user seems like the right balance to me.

Footnotes

1. https://github.com/ZcashFoundation/zebra/blob/main/zebra-test/src/lib.rs#L62-L143 ↩

[jaysonsantos]

@yaahc cool! Then, I just sent the fix for the example as I only ran a simple cargo test here

[yaahc]

I updated an error message and I think it's gonna break fmt :X

edit: it didn't! Easyyyyy

[jaysonsantos]

this also seems to fix #78