Customize HTML templates

[loay]

Customize html templates in strong-error-handler, see strongloop/loopback#1650 (comment) for details.

Configuration options:

// map of templates to use for rendering responses
views: {
 // the default view
 default: path.resolve(__dirname, 'views/error-default.jade'),
 404: path.resolve(__dirname, 'views/error-not-found.jade'),
 // etc.
},

// map of static files to send as a response
files: {
 // default: path.resolve(__dirname, 'public/error.html'),
 401: path.resolve(__dirname, 'views/error-unauthorized.html'),
 // etc.
},

Requires #5