contracts: Allow to distinguish out of gas from other traps

When a contract encounters a runtime error a wasm trap is
triggered and the execution is halted. Currently, no matter
what was the cause for the trap it is always reported as:
DispatchError::Other("contract trapped during execution").

However, the trap that is triggered if a contract exhausts
its gas budget is particulary interesting. Therefore we add
a seperate error message for this cause:
DispatchError::Other("ran out of gas during contract execution").

A test is added hat executes a contract that never terminates.
Therefore it always exhausts is gas budget.