wallet: Improve log output for errors during load

When loading the wallet, display the entire path in error messages, instead of
the name (which, for the default wallet, is the empty string.)

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

src/wallet/walletdb.cpp

@@ -422,8 +422,15 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
                 strType != "minversion" && strType != "acentry") {
             wss.m_unknown_records++;
         }
-    } catch (...)
-    {
+    } catch (const std::exception& e) {
+        if (strErr.empty()) {
+            strErr = e.what();
+        }
+        return false;
+    } catch (...) {
+        if (strErr.empty()) {
+            strErr = "Caught unknown exception in ReadKeyValue";
+        }
         return false;
     }
     return true;