Use logException consistently and use logDiagnostic

Many places around were re-inventing logException with varying level of success.
In addition, the full error is now printed as a diagnostic instead of debug.
This is more in line with the description of `LogLevel.diagnostic`
("Extended user information (e.g. for more detailed error information)"),
as opposed to the one of `LogLevel.debug_`
("Developer information useful for algorithm debugging").

tests/0-tcpproxy.d

@@ -112,8 +112,7 @@ int main()
 	runTask({
 		try runTest();
 		catch (Throwable th) {
-			logError("Test failed: %s", th.msg);
-			logDiagnostic("Full error: %s", th);
+			th.logException("Test failed");
 			ret = 1;
 		} finally exitEventLoop(true);
 	});