Display a better page for users when we fail to parse the workflow

## Description

From #394 , I think some workflows are failing as the tool is not able to find the commit in the repo (need to debug it to confirm.)

And I think those workflows create unnecessary errors 5xx's. We can do better, displaying some useful message to the user (e.g. "We could not load the selected workflow: git commit not found" and maybe a link to repo & to the commit.)

## Expected Behavior

If the workflow is found, i.e. it exists in our DB, then we return a 2xx page, with either the workflow contents (graph/description/etc) or a message explaining why we couldn't render it.

## Actual Behavior

Error 5xx.

## Possible Fix

## Steps to Reproduce

1.
2.
3.
4.

## Context

## Your Environment

* Version used:
* Environment name and version (e.g. Chrome 39, Java 1.8):