manifest: improve ManifestImportError __str__

Try to provide more helpful text.

Signed-off-by: Martí Bolívar <[email protected]>

src/west/app/main.py

@@ -843,8 +843,9 @@ def add_argument(self, *args, **kwargs):
     def error(self, message):
         if self.west_app and self.west_app.mle:
             # If we have a known WestApp instance and the manifest
-            # failed to load, then try to specialize the error message
-            # to handle west-specific situations.
+            # failed to load, then try to specialize the generic error
+            # message we're getting from argparse to handle west-specific
+            # errors better.
 
             app = self.west_app
             mle = self.west_app.mle

src/west/manifest.py

@@ -565,12 +565,14 @@ def __init__(self, project: Optional['Project'], imp: Any):
         self.imp = imp
 
     def __str__(self):
-        if self.project is not None:
-            return (f'ManifestImportFailed: project {self.project} '
-                    f'value {self.imp}')
+        if self.project is None:
+            # This happens when imports fail in the manifest repository
+            return (f'cannot import {self.imp}; is it present '
+                    'in your manifest repository?')
         else:
-            return (f'ManifestImportFailed: manifest repository '
-                    f'value {self.imp}')
+            return (f'project {self.project.name_and_path}: '
+                    f'cannot import contents of {self.imp}; '
+                    'do you need to run "west update"?')
 
 
 class ManifestVersionError(Exception):