Emit export errors to stderr to prevent invalid output

An error in export could emit error messages to stdout, causing the requirements.txt output to stdout to be invalid.

Fixes python-poetry#4109

poetry/console/commands/export.py

@@ -41,7 +41,7 @@ def handle(self) -> None:
 
  locker = self.poetry.locker
  if not locker.is_locked():
- self.line("<comment>The lock file does not exist. Locking.</comment>")
+ self.line_error("<comment>The lock file does not exist. Locking.</comment>")
  options = []
  if self.io.is_debug():
  options.append(("-vvv", None))
@@ -53,7 +53,7 @@ def handle(self) -> None:
  self.call("lock", " ".join(options))
 
  if not locker.is_fresh():
- self.line(
+ self.line_error(
  "<warning>"
  "Warning: The lock file is not up to date with "
  "the latest changes in pyproject.toml. "