Improve error message for unmatched input

[tsandall]

If the parser fails to find a matching rule, it stops immediately and returns an error message as follows:

1 error occurred: /Users/torin/test.rego:3: no match found, unexpected ','

It would be nice if the error message contained more context such as the surrounding text.

In some cases, the error message is slightly misleading, for instance:

test.rego:

package ex

limit :- input.value >= .5

error:

1 error occurred: /Users/torin/test.rego:3: no match found, unexpected '>'

The floating point number doesn't parse however because of how expressions are matched, it fails at >.

It's possible the parser rules could be restructured to improve error handling however this seems like something the parser should be able to deal with.

Creating this ticket as a placeholder for more investigation into improvements.

[tsandall]

We can improve error messages by implementing code blocks as described here: mna/pigeon#18 (comment).