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/bitcoind.cpp

@@ -39,10 +39,15 @@ bool AppInit(int argc, char* argv[])
         ParseParameters(argc, argv);
         if (!boost::filesystem::is_directory(GetDataDir(false)))
         {
-            fprintf(stderr, "Error: Specified directory does not exist\n");
-            Shutdown();
+            fprintf(stderr, "Error: Specified data directory \"%s\" does not exist.\n", mapArgs["-datadir"].c_str());
+            return false;
         }
         ReadConfigFile(mapArgs, mapMultiArgs);
+        // Check for -testnet or -regtest parameter (TestNet() calls are only valid after this clause)
+        if (!SelectParamsFromCommandLine()) {
+            fprintf(stderr, "Error: Invalid combination of -regtest and -testnet.\n");
+            return false;
+        }
 
         if (mapArgs.count("-?") || mapArgs.count("--help"))
         {
@@ -67,10 +72,6 @@ bool AppInit(int argc, char* argv[])
 
         if (fCommandLine)
         {
-            if (!SelectParamsFromCommandLine()) {
-                fprintf(stderr, "Error: invalid combination of -regtest and -testnet.\n");
-                return false;
-            }
             int ret = CommandLineRPC(argc, argv);
             exit(ret);
         }