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/test/timedata_tests.cpp

@@ -65,7 +65,7 @@ BOOST_AUTO_TEST_CASE(addtimedata)
         MultiAddTimeData(1, DEFAULT_MAX_TIME_ADJUSTMENT + 1); //filter size 5
     }
 
-    BOOST_CHECK(GetWarnings("gui").find("clock is wrong") != std::string::npos);
+    BOOST_CHECK(GetWarnings(true).find("clock is wrong") != std::string::npos);
 
     // nTimeOffset is not changed if the median of offsets exceeds DEFAULT_MAX_TIME_ADJUSTMENT
     BOOST_CHECK_EQUAL(GetTimeOffset(), 0);