Update hint - errors when importing packages

[benkoshy]

Quick Summary:

• I was trying to import a package and I received this message here:

• What does this mean? The message is helpful but does not pose the solution required: how to import or install the library?
• This PR goes a small way towards alleviating that problem. I feel that this is in keeping with the goals of the elm compiler: clear error messages, and clear solutions to the problems. For the advanced users, the solution may be obvious -- not so for others.
• Note: this might not be the best solution. "You can install packages like this: elm install elm/http". we are hard coding the solution: elm install elm/http. perhaps the compiler should be smart enough to suggest the correct module to install?
• As always, these PRs are merely ideas for consideration. If they do not align with the goals of the library etc. feel free to close without compunction.

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Ben