Frappe errors show up in modal

[nikochiko]

Describe the bug
Frappe uses msgprint to send errors sometimes and that by default shows in a modal on the page. This isn't always desirable or foreseen as the method to show modal is not called directly from code written for some method.

This was one of the concerns in #86

To Reproduce
Steps to reproduce the behavior: #86 (comment)

Expected behavior
Some errors should go to the modal while some may need handling.

Screenshots

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.