commands: Rework newQemuHelper to bubble-up error instead of panicing

Rather than panicing on error, bubble up an error so that the execution
may recover correctly and the error shown to the user.

Fixes: #413
Signed-off-by: Christopher Obbard <chris.obbard@collabora.com>

commands.go

@@ -209,7 +209,11 @@ func (cmd *Command) restoreResolvConf(sum *[sha256.Size]byte) error {
 }
 
 func (cmd Command) Run(label string, cmdline ...string) error {
- q := newQemuHelper(cmd)
+ q, err := newQemuHelper(cmd)
+ if err != nil {
+ return err
+ }
+
  q.Setup()
  defer q.Cleanup()
 
@@ -283,11 +287,11 @@ type qemuHelper struct {
  qemutarget string
 }
 
-func newQemuHelper(c Command) qemuHelper {
+func newQemuHelper(c Command) (*qemuHelper, error) {
  q := qemuHelper{}
 
  if c.Chroot == "" || c.Architecture == "" {
- return q
+ return &q, nil
  }
 
  switch c.Architecture {
@@ -306,14 +310,14 @@ func newQemuHelper(c Command) qemuHelper {
  case "amd64", "i386":
  /* Dummy, no qemu */
  default:
- log.Panicf("Don't know qemu for Architecture %s", c.Architecture)
+ return nil, fmt.Errorf("Don't know qemu for architecture %s", c.Architecture)
  }
 
  if q.qemusrc != "" {
  q.qemutarget = path.Join(c.Chroot, q.qemusrc)
  }
 
- return q
+ return &q, nil
 }
 
 func (q qemuHelper) Setup() error {