[fix] allow asset importers to define which content they can load

[YBadiss]

Ethereum plugins as defined in https://github.com/LedgerHQ/ledger-asset-dapps/ don't necessarily have a b2c.json file anymore, in which case they should just be ignored by the Live for now as they are only defined for B2B.

The ethereum-plugins.js importer fails to load the plugin in question, and raises an exception that is properly caught and logged by the main script.
However the error can be scary, and does not need to be raised in this case, so I added the notion of shouldLoad which allows importers to decide whether to skip a file or not. This notion already existed implicitly (checking if the file is a JSON), and I have simply extended it.

Let me know if this implementation suits you =]

[codecov]

Codecov Report

Merging #772 (80d8b7b) into master (97d487f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #772   +/-   ##
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  Coverage   43.51%   43.51%           
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  Files          81       81           
  Lines        4883     4883           
  Branches      859      859           
=======================================
  Hits         2125     2125           
  Misses       2744     2744           
  Partials       14       14           

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