Make external module read loop more robust

[acammack-r7]

Changes from a "hope we get at most one message at a time" model to
something beginning to resemble a state machine. Also logs error output
and fails the MSF module when the external module fails.

Verification

• Your favorite external module (either haraka or capture_test.py should still work.
• Anything logged to stderr from the external module should show up in ~/.msf4/logs/framework.log

[mkienow-r7]

Verified using capture_test.py. Note, the first execution after removing the time.sleep(5) statement I observed one of the expected output statements missing. I was unable to reproduce this issue on subsequent tests.

msf > use auxiliary/server/capture/capture_test
msf auxiliary(capture_test) > run
[*] Auxiliary module running as background job 0.

[*] Starting server...
msf auxiliary(capture_test) > [*] TEST
[*] TEST
[*] TEST
[*] TEST

msf auxiliary(capture_test) >
msf auxiliary(capture_test) >
msf auxiliary(capture_test) >
msf auxiliary(capture_test) > run
[*] Auxiliary module running as background job 1.

[*] Starting server...
msf auxiliary(capture_test) > [*] TEST
[*] TEST
[*] TEST
[*] TEST
[*] TEST