call out subprocess termination from signals

when hitting Ctrl+C in a 'docker run bob …', the main script itself actually can't be terminated by Ctrl+C, because it's PID 1

in that case, rather than just shutting down silently (by terminating itself using a SIGINT), we will see the subprocess exit (because Ctrl+C is sent to the whole process group)

When this happens, the return code of the process will be negative, to indicate that it didn't exit with that code, but instead got terminated by a signal of that (absolute) number

bob/models.py

@@ -3,6 +3,7 @@
 import os
 import re
 import shutil
+import signal
 import sys
 from tempfile import mkstemp, mkdtemp
 from subprocess import Popen
@@ -138,9 +139,18 @@ class Formula(object):
 
         p.wait()
 
-        if p.returncode != 0:
+        if p.returncode > 0:
             print_stderr('Formula exited with return code {}.'.format(p.returncode), title='ERROR')
             sys.exit(1)
+        elif p.returncode < 0: # script was terminated by signal number abs(returncode)
+            signum = abs(p.returncode)
+            try:
+                # Python 3.5+
+                signame = signal.Signals(signum).name
+            except AttributeError:
+                signame = signum
+            print_stderr('Formula terminated by signal {}.'.format(signame), title='ERROR')
+            sys.exit(128+signum) # best we can do, given how we weren't terminated ourselves with the same signal (maybe we're PID 1, maybe another reason)
 
         print_stderr('\nBuild complete: {}'.format(self.build_path))