Create error class for internal errors

[sindresorhus]

From #359 (comment).

So we can disable the stack output for them.

Should checked for here: https://github.com/sindresorhus/ava/blob/e52db69261d9b15dd1b69c76d82875b66d024e6b/cli.js#L99

[ariporad]

@sindresorhus: Why not just do delete error.stack?

[sindresorhus]

@ariporad Seems cleaner to use an error subclass as it can be detected as such. Normal errors are expected to have a .stack property.