improvement: don't crash on unloadable plugins

In some cases, plugins declare an entry point but cannot be loaded.
This is the case when they depend on a version of tutor that is not the
one that is currently installed. This use case is very frequent when
working on multiple versions at the same time (i.e: right now, while we
are working on the Maple release). In such cases, it's best just to
ignore the plugin entirely rather than having to re-install all plugins
in the virtualenv.

CHANGELOG.md

@@ -4,6 +4,8 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Improvement] Ignore Python plugins which cannot be loaded.
+
 ## v12.1.6 (2021-11-02)
 
 - [Improvement] Upgrade all services to open-release/lilac.3.

tutor/plugins.py

@@ -231,14 +231,18 @@ class EntrypointPlugin(BasePlugin):
     def iter_load(cls) -> Iterator["EntrypointPlugin"]:
         for entrypoint in pkg_resources.iter_entry_points(cls.ENTRYPOINT):
             try:
+                error: Optional[str] = None
                 yield cls(entrypoint)
-            except:
+            except pkg_resources.VersionConflict as e:
+                error = e.report()
+            except Exception as e:
+                error = str(e)
+            if error:
                 fmt.echo_error(
-                    "Failed to load entrypoint '{} = {}' from distribution {}".format(
-                        entrypoint.name, entrypoint.module_name, entrypoint.dist
+                    "Failed to load entrypoint '{} = {}' from distribution {}: {}".format(
+                        entrypoint.name, entrypoint.module_name, entrypoint.dist, error
                     )
                 )
-                raise
 
 
 class OfficialPlugin(BasePlugin):