check if the slashing protection database is locked before creating keys (#1949)

## Issue Addressed

Closes #1790

## Proposed Changes

Make a new method that creates an empty transaction with `TransactionBehavior::Exclusive` to check whether the slashing protection is locked. Call this method before attempting to create or import new validator keystores.  

## Additional Info

N/A


Co-authored-by: realbigsean <seananderson33@gmail.com>

validator_client/slashing_protection/src/slashing_database.rs

@@ -136,6 +136,13 @@ impl SlashingDatabase {
     #[cfg(windows)]
     fn set_db_file_permissions(file: &File) -> Result<(), NotSafe> {}
 
+    /// Creates an empty transaction and drops it. Used to test whether the database is locked.
+    pub fn test_transaction(&self) -> Result<(), NotSafe> {
+        let mut conn = self.conn_pool.get()?;
+        Transaction::new(&mut conn, TransactionBehavior::Exclusive)?;
+        Ok(())
+    }
+
     /// Register a validator with the slashing protection database.
     ///
     /// This allows the validator to record their signatures in the database, and check
@@ -803,4 +810,14 @@ mod tests {
         let db2 = SlashingDatabase::open(&file).unwrap();
         check(&db2);
     }
+
+    #[test]
+    fn test_transaction_failure() {
+        let dir = tempdir().unwrap();
+        let file = dir.path().join("db.sqlite");
+        let _db1 = SlashingDatabase::create(&file).unwrap();
+
+        let db2 = SlashingDatabase::open(&file).unwrap();
+        db2.test_transaction().unwrap_err();
+    }
 }