Fix unassigned variable.

In the event of an error, 'req' will be unassigned. This will cause another error in the exception.

Replace 'req' with 'name.'

news/4811.bugfix

@@ -0,0 +1,2 @@
+Fix an issue where a variable assigned in a try clause was accessed in the except clause, resulting in an undefined
+variable error in the except clause.

src/pip/_internal/req/req_install.py

@@ -139,7 +139,7 @@ def from_editable(cls, editable_req, comes_from=None, isolated=False,
             try:
                 req = Requirement(name)
             except InvalidRequirement:
-                raise InstallationError("Invalid requirement: '%s'" % req)
+                raise InstallationError("Invalid requirement: '%s'" % name)
         else:
             req = None
         return cls(