Improve error reporting

[MichaelClerx]

For web lab to gain adoption, we need it to be extremely usable. Part of this means providing really helpful error messages if users upload invalid CellML files. The current messages could do with a lot of work, e.g. by adding line numbers, being clearer about the error, etc.

[MichaelClerx]

Part of this means rewriting error messages to talk about the CellML code, not about cellmlmanip internals