Consider using enhanced TOML error messages

**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](https://github.com/BurntSushi/toml/releases/tag/v1.0.0):
> 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:
- #587

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No source file or test is named. Start by tracing ytt's TOML decoding error path and reviewing the upgrade context in #587 alongside the BurntSushi/toml v1.0.0 release notes. Compare current output with ErrorWithPosition() and ErrorWithUsage(), then establish whether verbosity should be default or optional and document the resulting behavior.

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