Add error message when git returns non-0 for fetch

Users have complained that fetchGit is flaky however the culprit is
likely that `git fetch` was unable itself to download the repository for
whatever reason (i.e. poor network etc..)

Nothing was checking the status of `git fetch` and the error message
that would eventually surface to the users were that the commit was not
found.

Add explicit error checking for status code from `git fetch` and return
a message earlier on to indicate that the failure was from that point.

fixes #10431

src/libfetchers/git-utils.cc

@@ -545,7 +545,7 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
             append(gitArgs, {"--depth", "1"});
         append(gitArgs, {std::string("--"), url, refspec});
 
-        runProgram(RunOptions {
+        auto [status, output] = runProgram(RunOptions {
             .program = "git",
             .lookupPath = true,
             // FIXME: git stderr messes up our progress indicator, so
@@ -554,6 +554,10 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
             .input = {},
             .isInteractive = true
         });
+        
+        if (status > 0) {
+            throw Error("Failed to fetch git repository %s: %s", url, output);
+        }
     }
 
     void verifyCommit(