Remove duplication in wallets availability checks

This change adds a new 'verifyWalletsAreAvailable' method to the client,
which eliminates this duplicated statement:
    throw new IllegalStateException("wallet is not yet available");

The commit is in response to a requested change in PR 4312:
bisq-network#4312 (review)

core/src/main/java/bisq/core/grpc/CoreWalletsService.java

@@ -72,9 +72,7 @@ public CoreWalletsService(Balances balances,
     }
 
     public long getAvailableBalance() {
-        if (!walletsManager.areWalletsAvailable())
-            throw new IllegalStateException("wallet is not yet available");
-
+        verifyWalletsAreAvailable();
         verifyEncryptedWalletIsUnlocked();
 
         var balance = balances.getAvailableBalance().get();
@@ -96,9 +94,7 @@ public AddressBalanceInfo getAddressBalanceInfo(String addressString) {
     }
 
     public List<AddressBalanceInfo> getFundingAddresses() {
-        if (!walletsManager.areWalletsAvailable())
-            throw new IllegalStateException("wallet is not yet available");
-
+        verifyWalletsAreAvailable();
         verifyEncryptedWalletIsUnlocked();
 
         // Create a new funding address if none exists.
@@ -140,8 +136,7 @@ public int getNumConfirmationsForMostRecentTransaction(String addressString) {
     }
 
     public void setWalletPassword(String password, String newPassword) {
-        if (!walletsManager.areWalletsAvailable())
-            throw new IllegalStateException("wallet is not yet available");
+        verifyWalletsAreAvailable();
 
         KeyCrypterScrypt keyCrypterScrypt = getKeyCrypterScrypt();
 
@@ -230,6 +225,12 @@ public void removeWalletPassword(String password) {
         walletsManager.backupWallets();
     }
 
+    // Throws a RuntimeException if wallets are not available (encrypted or not).
+    private void verifyWalletsAreAvailable() {
+        if (!walletsManager.areWalletsAvailable())
+            throw new IllegalStateException("wallet is not yet available");
+    }
+
     // Throws a RuntimeException if wallets are not available or not encrypted.
     private void verifyWalletIsAvailableAndEncrypted() {
         if (!walletsManager.areWalletsAvailable())