import: intercept and rephrase OutputNotFound message

dvc/exceptions.py

@@ -338,3 +338,12 @@ class RemoteNotSpecifiedInExternalRepoError(DvcException):
             ),
             cause=cause,
         )
+
+
+class NoOutputInExternalRepoError(DvcException):
+    def __init__(self, repo_url, path):
+        super(NoOutputInExternalRepoError, self).__init__(
+            "Output '{}' not found in target repository: '{}'".format(
+                path, repo_url
+            )
+        )

dvc/repo/imp.py

@@ -1,6 +1,13 @@
+from dvc.exceptions import NoOutputInExternalRepoError
+from dvc.exceptions import OutputNotFoundError
+
+
 def imp(self, url, path, out=None, rev=None):
     erepo = {"url": url}
     if rev is not None:
         erepo["rev"] = rev
 
-    return self.imp_url(path, out=out, erepo=erepo, locked=True)
+    try:
+        return self.imp_url(path, out=out, erepo=erepo, locked=True)
+    except OutputNotFoundError:
+        raise NoOutputInExternalRepoError(url, path)