util: change GetWarnings parameter to bool

GetWarnings() changes the format of the output warning string based on a
passed-in string argument that can be set to "gui" or "statusbar".
Change the argument to a bool:

- there are only two types of behaviour, so a bool is a more natural
argument type
- changing the name to 'verbose' does not set any expectations for the
how the calling code will use the returned string (currently,
'statusbar' is used for RPC warnings, not a status bar)
- removes some error-handling code for when the passed-in string is not
one of the two strings expected.

src/rpc/mining.cpp

@@ -253,7 +253,7 @@ static UniValue getmininginfo(const JSONRPCRequest& request)
     obj.pushKV("networkhashps",    getnetworkhashps(request));
     obj.pushKV("pooledtx",         (uint64_t)mempool.size());
     obj.pushKV("chain",            Params().NetworkIDString());
-    obj.pushKV("warnings",         GetWarnings("statusbar"));
+    obj.pushKV("warnings",         GetWarnings(false));
     return obj;
 }