rpc: add custom error with code -100

Fixed getassetstate method: it should return error with code -100 when
asset with specified id is not found (as in c# node).

pkg/rpc/errors.go

@@ -62,6 +62,12 @@ func NewInternalServerError(data string, cause error) *Error {
 	return newError(-32603, http.StatusInternalServerError, "Internal error", data, cause)
 }
 
+// NewRPCError creates a new error with
+// code -100
+func NewRPCError(message string, data string, cause error) *Error {
+	return newError(-100, http.StatusUnprocessableEntity, message, data, cause)
+}
+
 // Error implements the error interface.
 func (e Error) Error() string {
 	return fmt.Sprintf("%s (%d) - %s - %s", e.Message, e.Code, e.Data, e.Cause)

pkg/rpc/server.go

@@ -234,7 +234,7 @@ Methods:
 		if as != nil {
 			results = wrappers.NewAssetState(as)
 		} else {
-			results = "Invalid assetid"
+			resultsErr = NewRPCError("Unknown asset", "", nil)
 		}
 
 	case "getaccountstate":

pkg/rpc/server_test.go

@@ -87,7 +87,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 		{
 			name:   "negative",
 			params: `["602c79718b16e442de58778e148d0b1084e3b2dffd5de6b7b16cee7969282de2"]`,
-			result: func(e *executor) interface{} { return "Invalid assetid" },
+			fail:   true,
 		},
 		{
 			name:   "no params",