Misleading error message when an image has an incorrect path

[ptgott]

Steps to reproduce:

1. Create a file called docs/pages/test.png in a content directory within your gravitational/docs clone.
2. Add the following content to the page:

 ---
 title: test
 description: test
 ---
 
 This is an image:
 
 ![my image](../fake.png)

1. Start the dev server via yarn dev and navigate to localhost:3000/docs/test

2. The error is:

   Error: Image with src "../fake.png" is missing required "width" property.
   

For a docs author who isn't familiar with this error, it can be difficult to debug. We should stat the file path of an image first to make sure it exists, and if not, show a more descriptive error.