Add header for openPMD-specific errors

[franzpoeschel]

This is just the scaffolding for the more fine-grained error-handling that we want to implement.
This adds:

• A class openPMD::Error which should serve as base class for all of openPMD's error types. It should not be instatiated directly, but via child classes.
• A namespace openPMD::error which bundles all those error types.
• An example error type openPMD::error::OperationUnsupportedInBackend, without using it.

Related: #983 (this is not a fix, but the fundament for a fix)

TODO:

• Python?

[ax3l]

Thanks a lot! 🎉