log(archival_state): Improve error message if node is already running

This error can be hit by a test if the client is already running. Let's
indicate that this is a possibility, so users don't delete this
directory for no good reason.

src/models/state/archival_state.rs

@@ -119,7 +119,8 @@ impl ArchivalState {
                 );
                 panic!(
                     "Could not open database; do not know how to proceed. Panicking.\n\
-                    If you suspect the database may be corrupted, consider renaming the directory {} or removing it altogether.",
+                    If you suspect the database may be corrupted, consider renaming the directory {}\
+                     or removing it altogether. Or perhaps a node is already running?",
                     ms_db_dir_path.display()
                 );
             }