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Cycles Network rendering, remove some exception throwing, replace with saner error handling

This patch adds a network_error() function more alike how other devices handle error's

- it adds a check for errors on load_kernels to make sure we do not crash if rendering without a server.
- it uses the non throwing variation of boost::asio::read.

Reviewers: brecht

Reviewed By: brecht

CC: brecht

Differential Revision: https://developer.blender.org/D86