Refresh coinbase key if the existing one was removed (#2493)

* Refresh coinbase key if the existing was removed

It happens if `wallet check` runs in parallel.
Alternative solution is not to remove it during the check.
Fixes #2412

wallet/src/libwallet/error.rs

@@ -104,7 +104,7 @@ pub enum ErrorKind {
 	CallbackImpl(&'static str),
 
 	/// Wallet backend error
-	#[fail(display = "Wallet store error")]
+	#[fail(display = "Wallet store error: {}", _0)]
 	Backend(String),
 
 	/// Callback implementation error conversion

wallet/src/libwallet/internal/updater.rs

@@ -473,7 +473,10 @@ where
 	let parent_key_id = wallet.parent_key_id();
 
 	let key_id = match key_id {
-		Some(key_id) => keys::retrieve_existing_key(wallet, key_id, None)?.0,
+		Some(key_id) => match keys::retrieve_existing_key(wallet, key_id, None) {
+			Ok(k) => k.0,
+			Err(_) => keys::next_available_key(wallet)?,
+		},
 		None => keys::next_available_key(wallet)?,
 	};