Improve error messages for misconfigured backend treatment (#7062)

* Add testing warning if misconfig cant find nx-loopback

* fix lazy_import TypeError while creating error message for ModuleNotFoundError

* Update wording based on comments in dispatch mtg

networkx/conftest.py

@@ -58,7 +58,17 @@ def pytest_configure(config):
         )
     else:
         backends = entry_points(name="nx-loopback", group="networkx.backends")
-    networkx.utils.backends.backends["nx-loopback"] = next(iter(backends))
+    if backends:
+        networkx.utils.backends.backends["nx-loopback"] = next(iter(backends))
+    else:
+        warnings.warn(
+            "\n\n             WARNING: Mixed NetworkX configuration! \n\n"
+            "        This environment has mixed configuration for networkx.\n"
+            "        The test object nx-loopback is not configured correctly.\n"
+            "        You should not be seeing this message.\n"
+            "        Try `pip install -e .`, or change your PYTHONPATH\n"
+            "        Make sure python finds the networkx repo you are testing\n\n"
+        )
 
 
 def pytest_collection_modifyitems(config, items):

networkx/lazy_imports.py

@@ -95,7 +95,7 @@ def __getattr__(self, x):
                 f"No module named '{fd['spec']}'\n\n"
                 "This error is lazily reported, having originally occurred in\n"
                 f'  File {fd["filename"]}, line {fd["lineno"]}, in {fd["function"]}\n\n'
-                f'----> {"".join(fd["code_context"]).strip()}'
+                f'----> {"".join(fd["code_context"] or "").strip()}'
             )