Make "Unreachable code?" a warning instead of a fatal error.

[supercom32]

Enhancement found using Dragon Basic commit ID d2ce042

When working with code, it is quite common to block off a section of code for debug purposes. However, if you do something like "Return 0", DBC is smart enough to detect that unreachable code has been found. It would be nice if DBC would log this as a warning instead of a fatal error, since it is a pain to manually comment out multiple lines of code, or find strange ways to fool the compiler into letting you get away with it.