[cros_vm]: Better error messages for qemu.

* If qemu-kvm is not installed, provide instructions to install.
* Provide better diagnostics for the case where --download-vm is not specified,
or a board without a downloadable VM is specified.

BUG=chromium:655280
TEST=manual

Change-Id: I5dbf9965770e42f6e80029bad8fe35b8461c52a7
Reviewed-on: https://chromium-review.googlesource.com/756748
Commit-Ready: Achuith Bhandarkar <achuith@chromium.org>
Tested-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_vm.py b/scripts/cros_vm.py
index 7c4e9b1..62b24da 100644
--- a/scripts/cros_vm.py
+++ b/scripts/cros_vm.py
@@ -118,14 +118,15 @@
     if not self.qemu_path:
       self.qemu_path = osutils.Which('qemu-system-x86_64')
     if not self.qemu_path:
-      raise VMError('qemu not found.')
+      raise VMError('qemu not found. Try: sudo apt-get install qemu')
     logging.debug('qemu path=%s', self.qemu_path)
 
     if not self.image_path:
       self.image_path = os.environ.get('VM_IMAGE_PATH', '')
     logging.debug('vm image path=%s', self.image_path)
     if not self.image_path or not os.path.exists(self.image_path):
-      raise VMError('VM image path %s does not exist.' % self.image_path)
+      raise VMError('No VM image path found. '
+                    'Use cros chrome-sdk --download-vm.')
 
     self._CleanupFiles(recreate=True)
     # Make sure we can read these files later on by creating them as ourselves.