[devmode] Handle errors and fallback for developer mode detection

In case the store client is not available, fall back to a simpler
way of detecting whether developer mode is installed or not.

When calls to enable / disable developer mode fail, report back the
error immediately instead of waiting for a reply that never comes.
Merge pull request #11 from thp/developer-mode-error-handling

[devmode] Handle errors and fallback for developer mode detection