bsd.port.mk: Handle incorrect binary aliases gracefully

Currently, if the src element of a BINARY_ALIAS pair is missing, the ports
framework would just print out the help message of install(1) and exit with a
failure. This might be hard to debug for novice ports users.

Let's introduce a check, which would allow us to print that the src component
is missing.

This situation happens more often than it seems to. For example, BINARY_ALIAS
tend to blow up when used for test dependencies, because binary aliases are
created before test dependencies are installed. This leads to a lot of
confusion for my experience.