Improve exception handling

[swissspidy]

This PR:

• accounts for the fact that the API returns a JSON object with an error property when a plugin is not available.
• Uses Composer's RemoteFileSystem class to handle downloads. It does retries, exception handling, etc.
• Displays a warning when getting a TransportException (which likely occurs when w.org is down).

To do:

• Allow passing RemoteFileSystem via constructor so we can use RemoteFilesystemMock in tests and verify the correctness of this functionality.
• Maybe only print this warning once, ideally after all updates/installs went through.

See #5.

[swissspidy]

@ocean90 When you've got a minute next week :-)

[ocean90]

Only noticed that the error message can be displayed twice if the API was down for both requests.

Updating dependencies (including require-dev)
Package operations: 0 installs, 1 update, 0 removals
Could not reach WordPress.org to verify plugin availability status.
Could not reach WordPress.org to verify plugin availability status.
  - Updating wpackagist-plugin/spotify-embed (0.1 => 0.2): Downloading (100%)
    Changelog: https://wordpress.org/plugins/spotify-embed/#developers
Writing lock file
Generating autoload files

[swissspidy]

Only noticed that the error message can be displayed twice if the API was down for both requests.

Yeah that's something I want to change next (see todo section in original PR description).