fix: better error display + stage providers' maintenance page

[ross-rosario]

• Another attempt at catching and printing sendgrid errors.
• Added ProvidersMaintenancePage. To be used later if needed.

[ross-rosario]

@FahdW we'll replace ProviderPage in polymath-issuer/src/routes.js with ProvidersMaintenancePage. Nothing fancy. But we won't do that now as it will hinder our ability to test.