build: Print stdout and stderr on error

Currently, if the "build" command specified in Capstanfile fails,
there's no way to see the error.  Fix that up by printing out stdout and
stderr on failures.

Example output looks as follows:

  [penberg@localhost http-server]$ capstan build
  Building http-server...
  cc -o http-server.so -std=gnu99 -fPIC -shared -levent http-server.c
  cc -o http-server -std=gnu99 -fPIC -levent http-server.c
  http-server.c:28:26: fatal error: event2/event.h: No such file or directory
   #include <event2/event.h>
                          ^
  http-server.c:28:26: fatal error: event2/event.h: No such file or directory
   #include <event2/event.h>
                          ^
  compilation terminated.
  compilation terminated.
  make: *** [http-server.so] Error 1
  make: *** Waiting for unfinished jobs....
  make: *** [http-server] Error 1

  exit status 2

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>