Rename Application.format_reason/1 to format_error

Makes the function name consistent with OTP modules, which all use `format_error` to format `reason` in `{:error, reason}`.

Examples:
* `:inet.format_error/1`
* `:file.format_error/1`
* `:ssl.format_error/1`

There are many more but they are all called `format_error/1`.