Fail when grype cant check for db update

Closes anchore#310

Signed-off-by: Shane Dell <[email protected]>

grype/db/curator.go

@@ -141,9 +141,9 @@ func (c *Curator) Update() (bool, error) {
 
  updateAvailable, metadata, updateEntry, err := c.IsUpdateAvailable()
  if err != nil {
- // we want to continue if possible even if we can't check for an update
  log.Warnf("unable to check for vulnerability database update")
  log.Debugf("check for vulnerability update failed: %+v", err)
+ return false, fmt.Errorf("unable to update vulnerability database: %w", err)
  }
  if updateAvailable {
  log.Infof("downloading new vulnerability DB")