update SelectParamsFromCommandLine() handling/order

- move SelectParamsFromCommandLine() from init.cpp to bitcoin.cpp to allow
  to use TestNet() for Bitcoin-Qt instead of GetBoolArg("-testnet", false)
- change order in bitcoind.cpp to match bitcoin.cpp functionality
- hamonize error message strings for missing datadir and failing
  SelectParamsFromCommandLine() in bitcoin.cpp and bitcoind.cpp
- use TestNet() call in splashscreen.cpp

src/qt/splashscreen.cpp

@@ -1,6 +1,7 @@
 #include "splashscreen.h"
 #include "clientversion.h"
 #include "util.h"
+#include "chainparams.h"
 
 #include <QApplication>
 #include <QPainter>
@@ -26,7 +27,7 @@ SplashScreen::SplashScreen(const QPixmap &pixmap, Qt::WindowFlags f) :
 
     // load the bitmap for writing some text over it
     QPixmap newPixmap;
-    if(GetBoolArg("-testnet", false)) {
+    if(TestNet()) {
         newPixmap     = QPixmap(":/images/splash_testnet");
     }
     else {