impl From<RenderError> for RenderErrorReason

[cmrschwarz]

This just allows one to get the RenderErrorReason out of a RenderError by value.
The current reason() function only gives a reference, and the reason member is not pub (presumably for good reason).

My usecase for this is that I am using render_to_write with known, good data.
So the only possible error is an std::io::Error from the underlying writer, so I want to return that.
Unfortunately, std::io::Error isn't Clone so I can't get it out of the &RenderErrorReason that I'm currently getting.

I figured this was simple enough to not require a huge discussion in an issue first, but I'll gladly implement a different solution for this usecase if this conversion function is not acceptable for some reason.

Cheers and thank you for the awesome library.

[coveralls]

coverage: 81.519% (-0.09%) from 81.61%
when pulling 5039170 on cmrschwarz:RenderErrorReason_from_RenderError
into 5efb0b6 on sunng87:master.