Capture GitCommandError in log file

changes/1232.misc.rst

@@ -0,0 +1 @@
+If `git` fails to update a template in the cookiecutter cache, the `git` command and output are now captured in the Briefcase log file.

src/briefcase/commands/base.py

@@ -772,10 +772,12 @@ def update_cookiecutter_cache(self, template: str, branch="master"):
                     # Attempt to update the repository
                     remote = repo.remote(name="origin")
                     remote.fetch()
-                except self.tools.git.exc.GitCommandError:
+                except self.tools.git.exc.GitCommandError as e:
                     # We are offline, or otherwise unable to contact
-                    # the origin git repo. It's OK to continue; but warn
-                    # the user that the template may be stale.
+                    # the origin git repo. It's OK to continue; but
+                    # capture the error in the log and warn the user
+                    # that the template may be stale.
+                    self.logger.debug(str(e))
                     self.logger.warning(
                         """
 *************************************************************************
@@ -789,6 +791,7 @@ def update_cookiecutter_cache(self, template: str, branch="master"):
 *************************************************************************
 """
                     )
+
                 try:
                     # Check out the branch for the required version tag.
                     head = remote.refs[branch]