Fix typos and improve error messages wording.

Fix some typos in comments and use better wording for the user-visible
errors.
Add line number to parse error messages.

Just saying "Failed to parse" was singularly unhelpful, at least mention at
which line number the error has happened. Perhaps column number reporting
could be added in the future, but the exact position of the error is not
always well-defined, so don't do it for now.

Replace all occurrences of "throw toml_parse_exception{...}" with the calls to
throw_parse_exception(...), this will make it easier to customize the error
reporting later if necessary by updating just this function.
Handle parsing errors in the example.

Aborting on invalid input is not especially user-friendly, even for a simple
example.
Merge pull request #3 from vadz/better-error-messages

Better error messages.