cleaner message when CLI can't load app

When loading the app fails for the --help command, only the error
message is shown, then the help text. The full traceback is shown for
other exceptions. Also show the message when loading fails while
getting a command, instead of only "command not found". The error
message goes to stderr to match other error behavior, and is in red
with an extra newline to make it more obvious next to the help text.

Also fixes an issue with the test_apps fixture that caused an imported
app to still be importable after the test was over and the path was
reset. Now the module cache is reset as well.

tests/test_cli.py

@@ -239,7 +239,7 @@ def test_locate_app_raises(test_apps, iname, aname):
         locate_app(info, iname, aname)
 
 
-def test_locate_app_suppress_raise():
+def test_locate_app_suppress_raise(test_apps):
     info = ScriptInfo()
     app = locate_app(info, "notanapp.py", None, raise_if_not_found=False)
     assert app is None
@@ -396,21 +396,36 @@ def test_flaskgroup_debug(runner, set_debug_flag):
     assert result.output == f"{not set_debug_flag}\n"
 
 
-def test_print_exceptions(runner):
-    """Print the stacktrace if the CLI."""
+def test_no_command_echo_loading_error():
+    from flask.cli import cli
 
-    def create_app():
-        raise Exception("oh no")
-        return Flask("flaskgroup")
+    runner = CliRunner(mix_stderr=False)
+    result = runner.invoke(cli, ["missing"])
+    assert result.exit_code == 2
+    assert "FLASK_APP" in result.stderr
+    assert "Usage:" in result.stderr
 
-    @click.group(cls=FlaskGroup, create_app=create_app)
-    def cli(**params):
-        pass
 
+def test_help_echo_loading_error():
+    from flask.cli import cli
+
+    runner = CliRunner(mix_stderr=False)
     result = runner.invoke(cli, ["--help"])
     assert result.exit_code == 0
-    assert "Exception: oh no" in result.output
-    assert "Traceback" in result.output
+    assert "FLASK_APP" in result.stderr
+    assert "Usage:" in result.stdout
+
+
+def test_help_echo_exception():
+    def create_app():
+        raise Exception("oh no")
+
+    cli = FlaskGroup(create_app=create_app)
+    runner = CliRunner(mix_stderr=False)
+    result = runner.invoke(cli, ["--help"])
+    assert result.exit_code == 0
+    assert "Exception: oh no" in result.stderr
+    assert "Usage:" in result.stdout
 
 
 class TestRoutes: