Deduplicate the message verifying code

The logic of verifying a message was duplicated in 2 places:

src/qt/signverifymessagedialog.cpp
  SignVerifyMessageDialog::on_verifyMessageButton_VM_clicked()

src/rpc/misc.cpp
  verifymessage()

with the only difference being the result handling. Move the logic into
a dedicated

src/util/message.cpp
  MessageVerify()

which returns a set of result codes, call it from the 2 places and just
handle the results differently in the callers.

src/test/util_tests.cpp

@@ -9,6 +9,7 @@
 #include <sync.h>
 #include <test/util/setup_common.h>
 #include <test/util/str.h>
+#include <util/message.h> // For MessageVerify()
 #include <util/moneystr.h>
 #include <util/strencodings.h>
 #include <util/string.h>
@@ -2025,4 +2026,56 @@ BOOST_AUTO_TEST_CASE(test_tracked_vector)
     BOOST_CHECK_EQUAL(v8[2].copies, 0);
 }
 
+BOOST_AUTO_TEST_CASE(message_verify)
+{
+    BOOST_CHECK_EQUAL(
+        MessageVerify(
+            "invalid address",
+            "signature should be irrelevant",
+            "message too"),
+        MessageVerificationResult::ERR_INVALID_ADDRESS);
+
+    BOOST_CHECK_EQUAL(
+        MessageVerify(
+            "3B5fQsEXEaV8v6U3ejYc8XaKXAkyQj2MjV",
+            "signature should be irrelevant",
+            "message too"),
+        MessageVerificationResult::ERR_ADDRESS_NO_KEY);
+
+    BOOST_CHECK_EQUAL(
+        MessageVerify(
+            "1KqbBpLy5FARmTPD4VZnDDpYjkUvkr82Pm",
+            "invalid signature, not in base64 encoding",
+            "message should be irrelevant"),
+        MessageVerificationResult::ERR_MALFORMED_SIGNATURE);
+
+    BOOST_CHECK_EQUAL(
+        MessageVerify(
+            "1KqbBpLy5FARmTPD4VZnDDpYjkUvkr82Pm",
+            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=",
+            "message should be irrelevant"),
+        MessageVerificationResult::ERR_PUBKEY_NOT_RECOVERED);
+
+    BOOST_CHECK_EQUAL(
+        MessageVerify(
+            "15CRxFdyRpGZLW9w8HnHvVduizdL5jKNbs",
+            "IPojfrX2dfPnH26UegfbGQQLrdK844DlHq5157/P6h57WyuS/Qsl+h/WSVGDF4MUi4rWSswW38oimDYfNNUBUOk=",
+            "I never signed this"),
+        MessageVerificationResult::ERR_NOT_SIGNED);
+
+    BOOST_CHECK_EQUAL(
+        MessageVerify(
+            "15CRxFdyRpGZLW9w8HnHvVduizdL5jKNbs",
+            "IPojfrX2dfPnH26UegfbGQQLrdK844DlHq5157/P6h57WyuS/Qsl+h/WSVGDF4MUi4rWSswW38oimDYfNNUBUOk=",
+            "Trust no one"),
+        MessageVerificationResult::OK);
+
+    BOOST_CHECK_EQUAL(
+        MessageVerify(
+            "11canuhp9X2NocwCq7xNrQYTmUgZAnLK3",
+            "IIcaIENoYW5jZWxsb3Igb24gYnJpbmsgb2Ygc2Vjb25kIGJhaWxvdXQgZm9yIGJhbmtzIAaHRtbCeDZINyavx14=",
+            "Trust me"),
+        MessageVerificationResult::OK);
+}
+
 BOOST_AUTO_TEST_SUITE_END()