Rename error() function

[VelocityRa]

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error is a really common name and if bitsery's user codebase defines it to a macro, bitsery won't compile.

Related to #48.

Feel free to close this if you think it's ridiculous :)

[fraillt]

Hello, It's not ridiculous, but that would be a breaking change... and at the moment I don't want to introduce it without any additional value :) On the other hand, I don't like current error handling mechanism, it's really limiting and doesn't allow custom error's from extensions. I would like it to replace with using standard types from <system_error>, but I don't like that it's a little heavy (brings in <string> and other issues), and that might restrict usage of the whole library in some environments. So currently I'm waiting for p1023 and in the future, I'll revisit it, and then I will be able to change the name as well :) So maybe this ticket can be left open, as a reminder, but I'm afraid that this is not something that will be resolved anytime soon...