Improve error handleing and messaging + opencast api integration

[ferishili]

This PR fixes #26

Description

The error message shown when opening a video/series is very blur and it is not descriptive.
Also I have found out that this plugin still uses the old api calls which have been depricate long time ago.

Solution

• The Opencast API (PHP Library) is now being used/integrated.
• The Error messages shown now got a bit more meaningful details, so that the user knows more about what is happening if there is an error in two following cases:
  • If the opencast is down or not accessible
  • If the publication to extract video url is not ready or the configuration is incorrect.
    We don't need more to describe to users apart from these 2 cases, as this plugin play the role of presenting the video to the end-users.

TO TEST

• Patch this PR
• Make sure you have a course with at least an upload video via block plugin!
• As we introduced the Opencast API integration, it is very helpful to test this PR throughly! means all functions as follows:
  • Check if the existing video in mod plugin still work/ behave as expected!
  • Try to create a new video with mod plugin.
  • Try to create a new series with mod plugin
• To see the newly added error messages:
  • temporary change the opencast credentials in tool plugin and then try to open an existing video in mod plugin
  • Run a long workflow process on an uploaded video and try to access/create mod video out of that during the process and check the error message!