Timing validation, finer error status

[psteckler]

In Transaction_logic, there were several places where the errors returned by validate_timing mapped to User_command_status.Failure.Source_insufficient_balance. The timing validation does check for sufficient funds, but the more important check is that a transaction does not take the balance below the calculated minimum balance, which depends on the timing parameters and the current global slot.

Add a new status, Source_minimum_balance_violation. To distinguish the two kinds of errors, give them distinct tags, and provide a function to map those tags to the appropriate status.

This distinction is mentioned in #5993.