Consider checking for the existence of files at time of download

[osrf-migration]

Original report (archived issue) by Nate Koenig (Bitbucket: Nathan Koenig).

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Summary

Potentially check if a file exists during the model:// to file:// conversion in LocalCache.cc::FixPaths. See https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-fuel-tools/pull-requests/82/ready-integration-with-ign-gazebo/activity#comment-89120270

Describe alternatives you've considered

We could report an error message, leave the model:// tag in it's original state, leave the file://, or do nothing.

Things to consider:

1. Should ign-fuel-tools take on the role of check for the existence of files, or is that the job of a downstream application?

2. Error messages of this type tend to clutter the console, and are not useful for downstream applications. Returning error codes are better.

3. There could be dependencies on other resources. So, a model that is converted from "model://" to "file://" may rely on addtional resources that will be downloaded in the future.