Show a helpful HTML error message when there are errors

[frankieroberto]

When there are syntax errors, it would be better to show a helpful error message as an HTML page in the browser rather than just relying on errors being reported in Terminal.

Errors could occur in:

• templates
• routing code (JavaScript)
• SASS compilation (CSS)
• other JavaScript code, eg custom filters
• session data defaults (JSON)

Ideally the error message would show the relevant code and highlight the lines where the error likely occurred, although this may not always be 100% accurate.