Handle errors in 'source' builtin.

- When file doesn't exist
- When no arguments are given

Fixes issue #23.

core/cmd_exec.py

@@ -174,8 +174,19 @@ def _Eval(self, argv):

  def _Source(self, argv):
    # TODO: Use LineReader
    with open(argv[0]) as f:
      code_str = f.read()
    try:
      path = argv[0]
    except IndexError:
      # TODO: Should point to the source statement that failed.
      util.error('source: missing required argument')
      return 1
    try:
      with open(path) as f:
        code_str = f.read()
    except IOError as e:
      # TODO: Should point to the source statement that failed.
      util.error('source %r failed: %s', path, os.strerror(e.errno))
      return 1
    return self._EvalHelper(code_str)

  def _Exec(self, argv):

spec/builtins.test.sh

@@ -69,6 +69,19 @@ echo 'LIBVAR=libvar' > $lib
echo $LIBVAR
# stdout: libvar

### Source nonexistent
source /nonexistent/path
echo status=$?
# stdout: status=1
# OK dash stdout: status=127

### Source with no arguments
source
echo status=$?
# stdout: status=1
# OK bash stdout: status=2
# OK dash stdout: status=127

### Exit builtin
exit 3
# status: 3