Failures to build due to missing relative images are confusing

[jbmorley]

If there's a relative image, InContext will attempt to replace tag src or srcset attributes with the absolute path by looking the path up in the database. If this file doesn't exist, the site build will fail:

• it's unclear whether it's right to fail builds this way; should it be a warning instead?
• the error message is incredibly unhelpful and you really have to know what's going on inside InContext to be able to understand it