cli: print failed git export reason for each ref

Not all reasons are actionable, but we print hint in common cryptic cases.

cli/src/cli_util.rs

@@ -1843,10 +1843,10 @@ pub fn print_failed_git_export(
     if !failed_branches.is_empty() {
         writeln!(ui.warning(), "Failed to export some branches:")?;
         let mut formatter = ui.stderr_formatter();
-        for failed_ref_export in failed_branches {
+        for FailedRefExport { name, reason } in failed_branches {
             formatter.write_str("  ")?;
-            write!(formatter.labeled("branch"), "{}", failed_ref_export.name)?;
+            write!(formatter.labeled("branch"), "{name}")?;
-            formatter.write_str("\n")?;
+            writeln!(formatter, ": {reason}")?;
         }
         drop(formatter);
         if failed_branches

cli/tests/test_branch_command.rs

@@ -74,7 +74,7 @@ fn test_branch_at_root() {
     insta::assert_snapshot!(stderr, @r###"
     Nothing changed.
     Failed to export some branches:
-      fred
+      fred: Ref cannot point to the root commit in Git
     "###);
 }
 

cli/tests/test_git_colocated.rs

@@ -403,7 +403,7 @@ fn test_git_colocated_conflicting_git_refs() {
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
     Failed to export some branches:
-      main/sub
+      main/sub: Failed to set: A lock could not be obtained for reference "refs/heads/main/sub"
     Hint: Git doesn't allow a branch name that looks like a parent directory of
     another (e.g. `foo` and `foo/bar`). Try to rename the branches that failed to
     export or their "parent" branches.

cli/tests/test_git_import_export.rs

@@ -69,7 +69,7 @@ fn test_git_export_conflicting_git_refs() {
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
     Failed to export some branches:
-      main/sub
+      main/sub: Failed to set: A lock could not be obtained for reference "refs/heads/main/sub"
     Hint: Git doesn't allow a branch name that looks like a parent directory of
     another (e.g. `foo` and `foo/bar`). Try to rename the branches that failed to
     export or their "parent" branches.

lib/src/git.rs

@@ -543,25 +543,33 @@ pub struct FailedRefExport {
 }
 
 /// The reason we failed to export a ref to Git.
-#[derive(Debug)]
+#[derive(Debug, Error)]
 pub enum FailedRefExportReason {
     /// The name is not allowed in Git.
+    #[error("Name is not allowed in Git")]
     InvalidGitName,
     /// The ref was in a conflicted state from the last import. A re-import
     /// should fix it.
+    #[error("Ref was in a conflicted state from the last import")]
     ConflictedOldState,
     /// The branch points to the root commit, which Git doesn't have
+    #[error("Ref cannot point to the root commit in Git")]
     OnRootCommit,
     /// We wanted to delete it, but it had been modified in Git.
+    #[error("Deleted ref had been modified in Git")]
     DeletedInJjModifiedInGit,
     /// We wanted to add it, but Git had added it with a different target
+    #[error("Added ref had been added with a different target in Git")]
     AddedInJjAddedInGit,
     /// We wanted to modify it, but Git had deleted it
+    #[error("Modified ref had been deleted in Git")]
     ModifiedInJjDeletedInGit,
     /// Failed to delete the ref from the Git repo
-    FailedToDelete(Box<gix::reference::edit::Error>),
+    #[error("Failed to delete: {0}")]
+    FailedToDelete(#[source] Box<gix::reference::edit::Error>),
     /// Failed to set the ref in the Git repo
-    FailedToSet(Box<gix::reference::edit::Error>),
+    #[error("Failed to set: {0}")]
+    FailedToSet(#[source] Box<gix::reference::edit::Error>),
 }
 
 #[derive(Debug)]