Provide helpful error messages

[greenrobot]

If I'm not wrong, the check(cond) does not result in helpful messages.

To be more friendly to users, we should tell a) what's wrong and b) how to fix it.

[Offtree]

Might want to consider using the dart lang assert instead of the home brewed check function assuming you'd only want to run these assertions in development mode.

[greenrobot]

@Offtree Thanks, we'll have to decide on a case by case basis what is more suitable.

And, I just realized that have to clarify the issue and propose:

• checkObxPtr in addition to checkObx; throwing when a pointer from objectbox is invalid (NULL in C)
• check on anything model related (aka defined by user) should tell a) what's wrong and b) how to fix it; e.g. check(_idPropIdx != -1)
• use assert for "paranoia" mode in debug mode to have additional check on internals

Not sure if the pure check is still needed after that.

[nalenz-objectbox]

check has now been fully removed and checkObxPtr was added, which also reports a message.