Consider using enhanced TOML error messages

[pivotaljohn]

Describe the problem/challenge you have
ytt's TOML library now includes a feature to provide more verbose error messages. Let's consider taking advantage of that.

From the v1.0.0 release notes:

Decode always return a toml.ParseError, which has three methods:

• Error() behaves as before and shows a single concise line with the error.
• ErrorWithPosition() shows the same error, but also shows the line the error occurred at, similar to e.g. clang or the Rust compiler.
• ErrorWithUsage() is the same as ErrorWithPosition(), but may also show a longer usage guidance message. This isn't always present (in which case it behaves identical to ErrorWithPosition()), but it should be present for most common mistakes and sources of confusion.

Describe the solution you'd like

• Explore whether we should just adopt the more verbose error handling: how would doing so affect the formatting of our error messages?
• If that's an improvement, consider whether we should always provide that error handling, or plumb through an (optional) keyword argument to enable that verbosity.

Anything else you would like to add:
The upgrade came about through:

• Build(deps): Bump github.com/BurntSushi/toml from 0.4.1 to 1.0.0 #587

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