Chainparams: Translations: DRY: options and error strings

Also remove SelectBaseParamsFromCommandLine and SelectParamsFromCommandLine

src/bitcoind.cpp

@@ -102,8 +102,10 @@ bool AppInit(int argc, char* argv[])
             return false;
         }
         // Check for -testnet or -regtest parameter (Params() calls are only valid after this clause)
-        if (!SelectParamsFromCommandLine()) {
-            fprintf(stderr, "Error: Invalid combination of -regtest and -testnet.\n");
+        try {
+            SelectParams(ChainNameFromCommandLine());
+        } catch(std::exception &e) {
+            fprintf(stderr, "Error: %s\n", e.what());
             return false;
         }