Show error message if ReadConfigFile fails

A runaway exception was raised if ReadConfigFile fails (usually
due to a parse error in bitcoin.conf). Show an error message instead.

Fixes #4013.

src/bitcoin-cli.cpp

@@ -27,7 +27,12 @@ static bool AppInitRPC(int argc, char* argv[])
         fprintf(stderr, "Error: Specified data directory \"%s\" does not exist.\n", mapArgs["-datadir"].c_str());
         return false;
     }
-    ReadConfigFile(mapArgs, mapMultiArgs);
+    try {
+        ReadConfigFile(mapArgs, mapMultiArgs);
+    } catch(std::exception &e) {
+        fprintf(stderr,"Error reading configuration file: %s\n", e.what());
+        return false;
+    }
     // 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");

src/bitcoind.cpp

@@ -70,7 +70,13 @@ bool AppInit(int argc, char* argv[])
             fprintf(stderr, "Error: Specified data directory \"%s\" does not exist.\n", mapArgs["-datadir"].c_str());
             return false;
         }
-        ReadConfigFile(mapArgs, mapMultiArgs);
+        try
+        {
+            ReadConfigFile(mapArgs, mapMultiArgs);
+        } catch(std::exception &e) {
+            fprintf(stderr,"Error reading configuration file: %s\n", e.what());
+            return false;
+        }
         // 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");

src/qt/bitcoin.cpp

@@ -507,7 +507,13 @@ int main(int argc, char *argv[])
                               QObject::tr("Error: Specified data directory \"%1\" does not exist.").arg(QString::fromStdString(mapArgs["-datadir"])));
         return 1;
     }
-    ReadConfigFile(mapArgs, mapMultiArgs);
+    try {
+        ReadConfigFile(mapArgs, mapMultiArgs);
+    } catch(std::exception &e) {
+        QMessageBox::critical(0, QObject::tr("Bitcoin"),
+                              QObject::tr("Error: Cannot parse configuration file: %1. Only use key=value syntax.").arg(e.what()));
+        return false;
+    }
 
     /// 7. Determine network (and switch to network specific options)
     // - Do not call Params() before this step