add error message for missing pytest-cov while running coverage (microsoft#24288)

this error message is more specific than the generic "doesn't recognize
argument `--cov=.`" and will help point users in the right direction to
knowing how to enable coverage.

Author: eleanorjboyd

python_files/vscode_pytest/__init__.py

@@ -67,6 +67,13 @@ def __init__(self, message):
 
 
 def pytest_load_initial_conftests(early_config, parser, args):  # noqa: ARG001
+    has_pytest_cov = early_config.pluginmanager.hasplugin("pytest_cov")
+    has_cov_arg = any("--cov" in arg for arg in args)
+    if has_cov_arg and not has_pytest_cov:
+        raise VSCodePytestError(
+            "\n \nERROR: pytest-cov is not installed, please install this before running pytest with coverage as pytest-cov is required. \n"
+        )
+
     global TEST_RUN_PIPE
     TEST_RUN_PIPE = os.getenv("TEST_RUN_PIPE")
     error_string = (