Show location of error in ActionView template error

[RobinDaugherty]

Fixes #461, fixes #462.

Previous to Rails 5.1, when an error was raised in a template, it included the location within the template in the backtrace. As of 5.1, the exception was replaced with an ActionView::Template::Error which provides file_name and line_number but does not include those in the backtrace. It does provide a cause which contains the real error and correct backtrace, but as of Better Errors 2.7.0 (#459) we stopped showing only the "cause" of an error. (And in the future we want to add navigation within the chain of error causes, see #464.)

SyntaxError and Haml::SyntaxError were handled specially by Better Errors, finding the filename and line number within the exception message and synthesizing a backtrace item based on those. The workaround for ActionView::Template::Error looks exactly like those.

The result is that the topmost backtrace item will be the location of the error within the template.