Show error toast on startup for malformed tasks (#62387)

Closes https://github.com/zed-industries/zed/issues/61208

Before, Zed showed no toasts on startup when tasks.json contained
malformed entries, also if there were two top-level arrays, the last one
was silently discarded without any toasts too.

The PR fixes both.

Release Notes:

- Fixed error toast not showing for malformed tasks.json