feat(doit): don't crash if json decode fails.

Instead, tell the delegate about it and return the error.

Fixes #33

src/mako/lib/mbuild.mako

@@ -363,6 +363,7 @@ match result {
     Result::MissingToken => println!("Missing Token"),
     Result::Failure(_) => println!("General Failure (Response doesn't print)"),
     Result::FieldClash(clashed_field) => println!("FIELD CLASH: {:?}", clashed_field),
+    Result::JsonDecodeError(err) => println!("Json failed to decode: {:?}", err),
     Result::Success(_) => println!("Success (value doesn't print)"),
 }
 % endif
@@ -744,7 +745,10 @@ if enable_resource_parsing \
 {
                         let mut json_response = String::new();
                         res.read_to_string(&mut json_response).unwrap();
-                        (res, json::from_str(&json_response).unwrap())
+                        match json::from_str(&json_response) {
+                            Ok(decoded) => (res, decoded),
+                            Err(err) => return Result::JsonDecodeError(err),
+                        }
                     }\
                     % if supports_download:
  else { (res, Default::default()) }\

src/rust/cmn.rs

@@ -1,5 +1,6 @@
 use std::marker::MarkerTrait;
 use std::io::{self, Read, Seek, Cursor, Write, SeekFrom};
+use std;
 
 use mime::{Mime, TopLevel, SubLevel, Attr, Value};
 use oauth2;
@@ -8,6 +9,8 @@ use hyper::header::{ContentType, ContentLength, Headers};
 use hyper::http::LINE_ENDING;
 use hyper::method::Method;
 
+use serde;
+
 /// Identifies the Hub. There is only one per library, this trait is supposed
 /// to make intended use more explicit.
 /// The hub allows to access all resource methods more easily.
@@ -90,6 +93,11 @@ pub trait Delegate {
         None
     }
 
+    /// Called whenever a server response could not be decoded from json.
+    /// It's for informational purposes only, the caller will return with an error
+    /// accordingly.
+    fn response_json_decode_error(&mut self, json_encoded_value: &str) {}
+
     /// Called whenever the http request returns with a non-success status code.
     /// This can involve authentication issues, or anything else that very much 
     /// depends on the used API method.
@@ -134,6 +142,10 @@ pub enum Result<T = ()> {
     /// An additional, free form field clashed with one of the built-in optional ones
     FieldClash(&'static str),
 
+    /// Shows that we failed to decode the server response.
+    /// This can happen if the protocol changes in conjunction with strict json decoding.
+    JsonDecodeError(serde::json::Error),
+
     /// Indicates an HTTP repsonse with a non-success status code
     Failure(hyper::client::Response),