Modernized package code

• Code updated to PSR-4 layout and PSR-2 coding style (thanks to @daniellienert)
• Use modern PHP constants and literals where possible, use class constant for object manager access
• Added new configuration option sentryClientIgnoreException to Flow exception rendering options
• Ignore exceptions with status code 401 or 403 by default in Sentry (but still log locally according to logException option)
• Better stacktraces for non Exceptions (a.k.a Error), thanks to @fnkr