Allow spin to run in sub-directories/easier to spot that your error is caused by being in a sub-directory

[betatim]

Sometimes I am in a sub-directory of my repository and run spin doc (or some other command).

Currently this fails:

$ spin docs --no-plot
Error: need valid configuration in [.spin.toml], [spin.toml], or [pyproject.toml]
See https://github.com/scientific-python/spin/blob/main/README.md

Usage: spin [OPTIONS] COMMAND [ARGS]...
Try 'spin --help' for help.

Error: No such command 'docs'.

In the first instance this makes me think "ah, it must be doc not docs", which also fails. Mostly because I only read the "No such command" part of the output.

Two ideas:

1. what about making it so that spin works from any sub-directory, like git?
2. can we have spin exit after printing the first two lines? That seems like the message people should read and the usage info + "no such command" output is a distraction/red herring.

[stefanv]

I thought about this a bit, and I see two options:

(a) We only provide a more helpful error message, such as "Did you mean to run this from ../..?"
(b) We add a utility function that a command can use to figure out where it was called from, and where the root is. This is necessary because not all commands can run in the current directory: spin test must be run from root, typically, while spin run can be called in the current directory without a problem.

[betatim]

Maybe improving the error message is a "no brainer" that would make life better already. Then we can still think about allowing running from sub-directories or not

[stefanv]

@betatim Please see #232