Process Hangs if cover is not a string

[juancarlosfarah]

Summary

If you pass something that is not a string as a cover option, the epub generation process will hang on the "Making Cover..." step.

Steps to Reproduce

• Pass an empty object {} as options.cover.

Expected Behaviour

Process returns an error saying that options.cover must be a string.

Actual Behaviour

Process hangs.

Proposed Solution

Check the type of the cover option.

if (!(cover instanceof String) && (typeof cover !== 'string')) {
   // handle error
}

[gonejack]

If we couldn't write enough code to handle invalid input, we should let it crashes to help people understand their problems then take their fixes quickly, it should crashes instead of hanging in my opinion.