Better error when using conda without an environment file

[stsewd]

In this build https://readthedocs.org/projects/test-builds/builds/16326429/ I'm using coda, but I didn't define the environment file in the config file, so it gave me a generic error

The error came from

readthedocs.org/readthedocs/doc_builder/python_environments.py

Lines 550 to 557 in 6752de1

	inputfile = codecs.open(
	os.path.join(
	self.checkout_path,
	self.config.conda.environment,
	),
	encoding='utf-8',
	mode='r',
	)

We should probably require using conda.environment when using build.tools.python = conda

[humitos]

We should probably require using conda.environment when using build.tools.python = conda

This sounds like the right approach to me as well! 💯

[humitos]

The other way should also be considered probably: defining conda.environment but using build.tools.python: "3.8". See #10047 (comment)

[humitos]

When working on this issue, we should also take a look at #10502 which is strongly related.

[humitos]

Something similar happens when the Git submodule does not exist: https://github.com/readthedocs/readthedocs.org/pull/10594/files#r1282428289 -- The user gets a weird error.

I noting this here so we don't forget to improve this error when working on this.

[humitos]

This is another issue related to better error messaging on the config file #8595