Initialize ErrorManager based on RUN_MODE environment variable.

[ameyp]

I'd like to deploy this in production, but don't want error backtraces to be readily available to anyone that encounters them.

[bohwaz]

Thanks for your PR, I changed slightly to make it more simple (true/false instead of production/development) and also expose other features of ErrorManager.

[ameyp]

Looks good to me, thank you!