Added err.context & err.help to stack traces

- When we handle errors in Ghost, we are supposed to use a pattern of supplying 3 messages:
   - message: what went wrong
   - context: details about why how or where the error happened
   - help: where the user can go to get help with this error
- We do this in many places and our JSON error handler and CLI error logging tools are designed to output this extra information
- However, stack traces, which start with message as the first line and then output the stack are totally missing this
- By injecting the additional messages into the stack once an error has been "ghostified" we should get clearer messages everywhere

Notes:
- I've additionally injected a "Stack Trace:" line that makes it easier to read the error vs the stack
- This code looks a little weird because the lines are inserted backwards, but that allows us to always to the insert at position 1 as per the comment,
   so we don't have to keep track of whether we already injected something or not

core/server/web/shared/middleware/error-handler.js

@@ -60,6 +60,32 @@ _private.createHbsEngine = () => {
     return engine.express4();
 };
 
+_private.updateStack = (err) => {
+    let stackbits = err.stack.split(/\n/g);
+
+    // We build this up backwards, so we always insert at position 1
+
+    if (process.env.NODE_ENV === 'production' || err.statusCode === 404) {
+        // In production mode, remove the stack trace
+        stackbits.splice(1, stackbits.length - 1);
+    } else {
+        // In dev mode, clearly mark the strack trace
+        stackbits.splice(1, 0, `Stack Trace:`);
+    }
+
+    // Add in our custom cotext and help methods
+
+    if (err.help) {
+        stackbits.splice(1, 0, `${err.help}`);
+    }
+
+    if (err.context) {
+        stackbits.splice(1, 0, `${err.context}`);
+    }
+
+    return stackbits.join('\n');
+};
+
 /**
  * Get an error ready to be shown the the user
  *
@@ -102,6 +128,8 @@ _private.prepareError = (err, req, res, next) => {
     // alternative for res.status();
     res.statusCode = err.statusCode;
 
+    err.stack = _private.updateStack(err);
+
     // never cache errors
     res.set({
         'Cache-Control': 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0'
@@ -263,7 +291,7 @@ function createHtmlDocument(status, message) {
        <pre>${status} ${body}</pre>\n
        </body>\n
        </html>\n`;
-        }
+}
 
 _private.HTMLErrorRenderer = (err, req, res, next) => { // eslint-disable-line no-unused-vars
     return res.send(createHtmlDocument(res.statusCode, err.stack));