fix(pypi): show overridden index urls in pypi download error

[dotoleeoak]

Closes #2985

Suppose invalid experimental_index_url_overrides in pip.parse is set like below.

pip.parse( experimental_index_url = "https://pypi.org/simple", experimental_index_url_overrides = {"mypy": "https://invalid.com"}, hub_name = "pypi", requirements_lock = "//:requirements_lock.txt", )

It fails as follows, showing only "pypi.org" as pypi index url, not "invalid.com" for for mypy package.

Error in fail: Failed to download metadata for ["mypy"] for from urls: ["https://pypi.org/simple"]. If you would like to skip downloading metadata for these packages please add 'simpleapi_skip=["mypy"]' to your 'pip.parse' call. 

To show overridden url for each package, show url of each package that has been failed to download metadata. The error message with this PR is like below.

Error in fail: Failed to download metadata of the following packages from urls: { "mypy": "https://invalid.com", } If you would like to skip downloading metadata for these packages please add 'simpleapi_skip=["mypy"]' to your 'pip.parse' call. 

[aignas]

Thank you!