Add error handling for failed package installation

[vadi2]

mpkg repo add demo https://raw.githubusercontent.com/vadi2/MudletPackageManager/demo-
package/repository.json blah
Added new repository to database:
  name: demo
  URL: https://raw.githubusercontent.com/vadi2/MudletPackageManager/demo-package/repository.json
  Description: blah
mpkg update
Updating the list of available packages.
Package list of repository demo updated.
Waiting for 1 more repository updates to finish.
Package list of repository MudletPackageManager updated.
Finished updating package list.
mpkg package list
List of all known packages:
  MudletPackageManager (0.0.2) from MudletPackageManager installed
  one-click-install-demo (1) from demo 
mpkg install one-click-install-demo
Installing package one-click-install-demo.

[ERROR:] object:<event handler function> function:<mpkg.finishedDownload>
         <[string "Script: mpkg.finishedDownload"]:60: attempt to index local 'package' (a nil 
value)>