Added some in-application messages.

Help message for command-line arguments are added. Plus, some error
message shown during interpreter running are also modified.
bfi now terminates when a BF program causes access violation.

Access violation is like segmentation fault. By using this behavior,
bfi now have mor similarity with C and most other programs.

The following code causes access violation, thus make bfi terminate:

<<<<++

(Some implementation will move data pointer to tail of the data array
when such instructions are given.)