Improve error messaging when invalid syntax is passed to the -m option

changelog/4051.trivial

@@ -0,0 +1 @@
+Imrpove Error messaging when Invalid Python Syntax is passed through the ``-m`` option to limit what marks to run against.

src/_pytest/mark/legacy.py

@@ -66,7 +66,10 @@ python_keywords_allowed_list = ["or", "and", "not"]
 
 def matchmark(colitem, markexpr):
     """Tries to match on any marker names, attached to the given colitem."""
-    return eval(markexpr, {}, MarkMapping.from_item(colitem))
+    try:
+        return eval(markexpr, {}, MarkMapping.from_item(colitem))
+    except SyntaxError as e:
+        raise SyntaxError(str(e) + "\nMarker expression must be valid Python!")
 
 
 def matchkeyword(colitem, keywordexpr):

testing/test_mark.py

@@ -786,6 +786,18 @@ class TestFunctional(object):
         deselected_tests = dlist[0].items
         assert len(deselected_tests) == 2
 
+    def test_invalid_m_option(self, testdir):
+        testdir.makepyfile(
+            """
+            def test_a():
+                pass
+        """
+        )
+        result = testdir.runpytest("-m bogus/")
+        result.stdout.fnmatch_lines(
+            ["INTERNALERROR> Marker expression must be valid Python!"]
+        )
+
     def test_keywords_at_node_level(self, testdir):
         testdir.makepyfile(
             """