Theme: good error messages

[AustinWise]

When an error happens, it should be clear what the cause was and how the user can fix the problem. This includes both command ling error messages and error messages sent over HTTP.

• Error messages sent of HTTP should ideally look relatively nice. Like they should not be plain text; they should not be the output from a Debug trait impl.
• Do we have to worry about that thing where IE only displays error pages greater than 512 bytes in size? It looks like Chrome may have copied this feature
• Audit the different From implementations for MyError. Make sure the message matches the level of specificity of the error type they convert from.

Some specific cases that are known to not have good error messages:

• When a markdown file is not valid UTF-8.
• Failures to read wiki pages.
• Failure to find wiki pages.