Exported requirements.txt contains spurious error message, making it unparseable

[colindean]

• I am on the latest Poetry version.

• I have searched the issues of this repo and believe that this is not a duplicate.

• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: macOS 10.15.7

• Poetry version: 1.1.6

• Link of a Gist with the contents of your pyproject.toml file: Too much internal stuff to share, it's not really related, though

Issue

When I run poetry export > requirements.txt && pip install -r requirements.txt, I see this:

 Invalid requirement: 'Warning: The lock file is not up to date with the latest changes in pyproject.toml. You may be getting outdated dependencies. Run update to update them.' (from line 1 of requirements.txt)

It appears that my Poetry lockfile is out of sync. No biggie, that's correctable and logged in #3092 when it's not necessary to resync.

However, this error message should not make it into the output of the poetry export command. That error log line should go to stderr, not stdout.

Proposed Fix

I think that the self.line here

poetry/poetry/console/commands/export.py

Lines 56 to 63 in b753aaf

	self.line(
	"<warning>"
	"Warning: The lock file is not up to date with "
	"the latest changes in pyproject.toml. "
	"You may be getting outdated dependencies. "
	"Run update to update them."
	"</warning>"
	)

should be self.line_error which, according to the docstring for that method outputs to stderr instead of stdout.

[colindean]

Thank you!

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