Emit export errors to stderr to prevent invalid output (#4110)

* 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 #4109

* Looks at stderr for export tests

poetry/console/commands/export.py

@@ -41,7 +41,7 @@ class ExportCommand(Command):
 
         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 @@ class ExportCommand(Command):
             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. "

tests/console/commands/test_export.py

@@ -77,12 +77,12 @@ foo==1.0.0
 def test_export_exports_requirements_txt_file_locks_if_no_lock_file(tester, poetry):
     assert not poetry.locker.lock.exists()
     _export_requirements(tester, poetry)
-    assert "The lock file does not exist. Locking." in tester.io.fetch_output()
+    assert "The lock file does not exist. Locking." in tester.io.fetch_error()
 
 
 def test_export_exports_requirements_txt_uses_lock_file(tester, poetry, do_lock):
     _export_requirements(tester, poetry)
-    assert "The lock file does not exist. Locking." not in tester.io.fetch_output()
+    assert "The lock file does not exist. Locking." not in tester.io.fetch_error()
 
 
 def test_export_fails_on_invalid_format(tester, do_lock):