Uniform use of VmError for exceptions

[s1na]

Two minor points about how the VM handles exceptions:

• In most parts of the evm, exceptions are wrapped in a VmError. There are places however where this is not the case (e.g. in precompiles) and only the error message is returned as a string. This can be made uniform so that all exceptions are wrapped in a VmError.
• Result objects include two keys: exception and exceptionError. This is redundant. It can be checked whether an exception occured by checking whether exceptionError is null or contains a value.

[holgerd77]

Not sure what's the state of this, this would need some validation - optimally with some code links - or some comment by @s1na if/what of the above has been solved.

[alcuadrado]

I think this was fixed in #551