Improve error message for missing paths in mount operations

When mount() fails with ENOENT (No such file or directory), provide
a descriptive error message that includes:
- Source and destination paths in the error message
- System error description (strerror) and errno
- Helpful hint about checking source path existence

All other mount failures continue to use the generic check() assertion
as before, preserving the original error handling behavior for non-ENOENT
errors.

This makes debugging missing path issues much easier while maintaining
the existing error handling for other failure cases.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people

deps/userns_common.c

@@ -299,7 +299,15 @@ void bind_mount(const char *src, const char *dest, char read_only) {
   // We don't expect workspaces to have any submounts in normal operation.
   // However, for runshell(), workspace could be an arbitrary directory,
   // including one with sub-mounts, so allow that situation with MS_REC.
-  check(0 == mount(resolved_src, dest, "", MS_BIND|MS_REC, NULL));
+  int mount_result = mount(resolved_src, dest, "", MS_BIND|MS_REC, NULL);
+  if (mount_result != 0 && errno == ENOENT) {
+    // Provide a more descriptive error for missing paths
+    fprintf(stderr, "ERROR: Failed to mount '%s' to '%s': %s (errno %d)\n",
+            resolved_src, dest, strerror(errno), errno);
+    fprintf(stderr, "  Please ensure that '%s' exists and is accessible.\n", resolved_src);
+    _exit(1);
+  }
+  check(mount_result == 0);
 
   // remount to read-only. this requires a separate remount:
   // https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/?id=9ac77b8a78452eab0612523d27fee52159f5016a