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/interfaces/node.cpp

@@ -68,7 +68,7 @@ public:
     std::string getNetwork() override { return Params().NetworkIDString(); }
     void initLogging() override { InitLogging(); }
     void initParameterInteraction() override { InitParameterInteraction(); }
-    std::string getWarnings() override { return GetWarnings("gui"); }
+    std::string getWarnings() override { return GetWarnings(true); }
     uint32_t getLogCategories() override { return LogInstance().GetCategoryMask(); }
     bool baseInitialize() override
     {