Add fallback strategy for source selection - #890

Related issue: #872

Changes in this PR:

Implemented source fallback functionality for VideoJS,

• for source selection with the @silvermine/videojs-quality-selector plugin. With this fix, when a source choice in quality selector fails to load, the player automatically cycles through available sources until one works. While trying available source choices it marks unavailable sources choices in the quality selector menu. If all sources fail an error message is displayed using VideoJS' error modal.
• for a multi-sourced Canvas where multiple segments are stitched together to build a single Canvas. With this fix; when a source fails to load while user is scrubbing/using structured navigation actions, the next viable available segment is loaded into the player. And the failed segment is marked as blocked in the progress-bar for the player session.

When the player encounters a failing source that particular source gets disabled in the quality selector for the player session (until page is reloaded, or switched to another Canvas) and an error is logged to the devTools console.

When all sources fail an error is displayed using VideoJS's error modal.

When 2 consecutive source segments are failing in a multi-sourced Canvas the error message is displayed as a non-dismissable toast notification before loading the next viable available segment.