Create a custom error when plugin is not found. (#409)

Signed-off-by: Abdelhamid Bakhta <abdelhamid.bakhta@consensys.net>

ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/jsonrpc/internal/methods/PluginsReloadConfiguration.java

@@ -52,7 +52,7 @@ public class PluginsReloadConfiguration implements JsonRpcMethod {
             "Plugin cannot be reloaded because no plugin has been registered with specified name: {}.",
             pluginName);
         return new JsonRpcErrorResponse(
-            requestContext.getRequest().getId(), JsonRpcError.INTERNAL_ERROR);
+            requestContext.getRequest().getId(), JsonRpcError.PLUGIN_NOT_FOUND);
       }
       reloadPluginConfig(namedPlugins.get(pluginName));
       return new JsonRpcSuccessResponse(requestContext.getRequest().getId());

ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/jsonrpc/internal/response/JsonRpcError.java

@@ -150,7 +150,10 @@ public enum JsonRpcError {
   ENCLAVE_PRIVACY_GROUP_MISSING(-50200, "PrivacyGroupNotFound"),
   ENCLAVE_PRIVACY_QUERY_ERROR(-50200, "PrivacyGroupQueryError"),
   ENCLAVE_KEYS_CANNOT_DECRYPT_PAYLOAD(-50200, "EnclaveKeysCannotDecryptPayload"),
-  METHOD_UNIMPLEMENTED(-50200, "MethodUnimplemented");
+  METHOD_UNIMPLEMENTED(-50200, "MethodUnimplemented"),
+
+  /** Plugins error */
+  PLUGIN_NOT_FOUND(-60000, "Plugin not found");
 
   private final int code;
   private final String message;