Update Warning-strings to use a standard-format

- ensure warnings always start with "Warning:" and that the first
  character after ":" is written uppercase
- ensure the first sentence in warnings ends with an "!"
- remove unneeded spaces from Warning-strings
- add missing Warning-string translation
- remove a "\n" and replace with untranslatable "<br><br>"

src/wallet.cpp

@@ -1557,7 +1557,7 @@ CPubKey CReserveKey::GetReservedKey()
             vchPubKey = keypool.vchPubKey;
         else
         {
-            printf("CReserveKey::GetReservedKey(): Warning: using default key instead of a new key, top up your keypool.");
+            printf("CReserveKey::GetReservedKey(): Warning: Using default key instead of a new key, top up your keypool!");
             vchPubKey = pwallet->vchDefaultKey;
         }
     }