Improved error messaging

[kjcjohnson]

Make error handling better, because it's not particularly clear right now.

Plus a bug fix with default grammar generation...

[kjcjohnson]

Files updated: https://www.notion.so/Parser-Files-e44358c520214642929bafaf363ea321

Includes:

• Syntax constructors for CHCs in the verifier output (as reported by @ebmoon)

  CHC: (E.Sem et x y r) <= (E.Sem et1 x y r1) ^ (E.Sem et2 x y r1) ^ (= r (+ r1 r2))
  [constructor: ($+ et1 et2)]     [inputs: x y]     [outputs: r]

  (the [constructor ...] part is new)

• Primitive did-you-mean faculty to help with typos (as inspired by problems reported by @anvayg)

  error: string:69:16: Cannot find function definition for: B.sem
  Did you mean:
   - E.Sem
   - B.Sem