Improve error messages

[rjoleary]

• Post a style guide for warnings and errors. How validation warnings are dealt with (I know we squelch some of these warnings). When to log.Print vs log.Fatal, ...
• Go through the code and fix up our error messages.
• Particularly, the "file too large" error needs to be improved. It's probably the most common error to run into. It prints a lot of information, but it would print some pointers on next steps.