wallet: Improve log output for errors during load

When loading the default wallet, display the wallet name in error messages as
"[default wallet]", instead of the empty string which gives confusing messages.

When an exception occurs during wallet loading, display e.what() if possible,
instead of nothing.

src/wallet/wallet.cpp

@@ -4097,7 +4097,7 @@ bool CWallet::Verify(interfaces::Chain& chain, const WalletLocation& location, b
 
 std::shared_ptr<CWallet> CWallet::CreateWalletFromFile(interfaces::Chain& chain, const WalletLocation& location, uint64_t wallet_creation_flags)
 {
-    const std::string& walletFile = location.GetName();
+    const std::string& walletFile = (location.GetName() == "" ? "[default wallet]" : location.GetName());
 
     // needed to restore wallet transaction meta data after -zapwallettxes
     std::vector<CWalletTx> vWtx;

src/wallet/walletdb.cpp

@@ -422,8 +422,17 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
                 strType != "minversion" && strType != "acentry") {
             wss.m_unknown_records++;
         }
+    } catch (const std::exception& e)
+    {
+        if (strErr == "") {
+            strErr = e.what();
+        }
+        return false;
     } catch (...)
     {
+        if (strErr == "") {
+            strErr = "Caught unknown exception in ReadKeyValue";
+        }
         return false;
     }
     return true;