Increase signal to noise in appveyor build output by reducing the MSVC warning count from 12 to 4 (12 is assuming the changes in #14086 are also implemented).

This makes it easier to spot errors or more important warnings in the verbose appveyor output.

See https://github.com/bitcoin/bitcoin/pull/14086#issuecomment-416610313 plus discussion for context.

src/rest.cpp

@@ -464,7 +464,7 @@ static bool rest_getutxos(HTTPRequest* req, const std::string& strURIPart)
                 oss >> fCheckMemPool;
                 oss >> vOutPoints;
             }
-        } catch (const std::ios_base::failure& e) {
+        } catch (const std::ios_base::failure&) {
             // abort in case of unreadable binary data
             return RESTERR(req, HTTP_BAD_REQUEST, "Parse error");
         }

src/test/coins_tests.cpp

@@ -508,7 +508,7 @@ BOOST_AUTO_TEST_CASE(ccoins_serialization)
         Coin cc4;
         ss4 >> cc4;
         BOOST_CHECK_MESSAGE(false, "We should have thrown");
-    } catch (const std::ios_base::failure& e) {
+    } catch (const std::ios_base::failure&) {
     }
 
     // Very large scriptPubKey (3*10^9 bytes) past the end of the stream
@@ -521,7 +521,7 @@ BOOST_AUTO_TEST_CASE(ccoins_serialization)
         Coin cc5;
         ss5 >> cc5;
         BOOST_CHECK_MESSAGE(false, "We should have thrown");
-    } catch (const std::ios_base::failure& e) {
+    } catch (const std::ios_base::failure&) {
     }
 }
 
@@ -719,7 +719,7 @@ static void CheckAddCoinBase(CAmount base_value, CAmount cache_value, CAmount mo
         test.cache.AddCoin(OUTPOINT, Coin(std::move(output), 1, coinbase), coinbase);
         test.cache.SelfTest();
         GetCoinsMapEntry(test.cache.map(), result_value, result_flags);
-    } catch (std::logic_error& e) {
+    } catch (std::logic_error&) {
         result_value = FAIL;
         result_flags = NO_ENTRY;
     }
@@ -780,7 +780,7 @@ void CheckWriteCoins(CAmount parent_value, CAmount child_value, CAmount expected
         WriteCoinsViewEntry(test.cache, child_value, child_flags);
         test.cache.SelfTest();
         GetCoinsMapEntry(test.cache.map(), result_value, result_flags);
-    } catch (std::logic_error& e) {
+    } catch (std::logic_error&) {
         result_value = FAIL;
         result_flags = NO_ENTRY;
     }

src/test/dbwrapper_tests.cpp

@@ -262,7 +262,7 @@ struct StringContentsSerializer {
                 try {
                     READWRITE(c);
                     str.push_back(c);
-                } catch (const std::ios_base::failure& e) {
+                } catch (const std::ios_base::failure&) {
                     break;
                 }
             }

src/test/net_tests.cpp

@@ -115,7 +115,7 @@ BOOST_AUTO_TEST_CASE(caddrdb_read)
         unsigned char pchMsgTmp[4];
         ssPeers1 >> pchMsgTmp;
         ssPeers1 >> addrman1;
-    } catch (const std::exception& e) {
+    } catch (const std::exception&) {
         exceptionThrown = true;
     }
 
@@ -148,7 +148,7 @@ BOOST_AUTO_TEST_CASE(caddrdb_read_corrupted)
         unsigned char pchMsgTmp[4];
         ssPeers1 >> pchMsgTmp;
         ssPeers1 >> addrman1;
-    } catch (const std::exception& e) {
+    } catch (const std::exception&) {
         exceptionThrown = true;
     }
     // Even through de-serialization failed addrman is not left in a clean state.

src/wallet/rpcwallet.cpp

@@ -288,7 +288,6 @@ static UniValue setlabel(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
     }
 
-    std::string old_label = pwallet->mapAddressBook[dest].name;
     std::string label = LabelFromValue(request.params[1]);
 
     if (IsMine(*pwallet, dest)) {