Add a .to_json conversion for Error (#1742)

distribution/std-lib/Standard/src/Base/Error/Extensions.enso

@@ -84,6 +84,23 @@ Error.to_default_visualization_data = this.catch .to_default_visualization_data
 Error.to_display_text : Text
 Error.to_display_text  = "Error: " + (this.catch .to_display_text)
 
+## UNSTABLE
+
+   Returns a JSON representation of the dataflow error.
+
+   > Example
+     Converting a dataflow error to JSON.
+
+         import Standard.Examples
+
+         example_to_json = Examples.throw_error.to_json
+Error.to_json : Json.Object
+Error.to_json =
+    error_type = ["type", "Error"]
+    error_content = ["content", this.catch .to_json]
+    error_message = ["message", this.catch .to_display_text]
+    Json.from_pairs [error_type, error_content, error_message]
+
 ## Transforms an error.
 
    Arguments:
@@ -126,4 +143,3 @@ Error.is_error = True
          example_rethrow = Panic.rethrow Examples.throw_error
 Panic.rethrow : (Any ! Any) -> Any
 Panic.rethrow value = value.catch Panic.throw
-

test/Tests/src/Semantic/Error_Spec.enso

@@ -23,6 +23,21 @@ spec =
         Test.specify "should be able to be shown in the default visualization" <|
             json = (Error.throw <| My_Type "aaa").to_default_visualization_data
             json . should_equal <| (Json.from_pairs [["foo", "aaa"], ["type", "My_Type"]]).to_text
+
+        Test.specify "should be able to be shown in the default vector visualization" <|
+            vec = [My_Type "bar", Error.throw (My_Type 42)]
+            visualization_text = vec.to_default_visualization_data
+            expected_json = Json.parse '''
+                [
+                  { "foo":"bar",
+                    "type":"My_Type"
+                  },
+                  { "content":{ "foo":42, "type":"My_Type" },
+                    "message":"My_Type",
+                    "type":"Error"
+                  }
+                ]
+            visualization_text.should_equal expected_json.to_text
         Test.specify "should implement to_display_text" <|
             Error.throw Nothing . to_display_text . should_equal "Error: Nothing"
         Test.specify "should be able to be mapped" <|