[TM-267]: Fix bad error messages in let-block

Description

Problem: Bad error message in let-block

For example, if we have this in let-block:

let {
  type Store = (BigMap Address Nat, Nat);

  
  test :: '[option int] -> '[int]
  = { IF_SOME { nop } { PUSH int 3 }; };
  #             ^^^
  # Adding `nop` here will cause an error

};

Expected error message:

morley: ../../contracts/FA1/custom.mtz:10:16:
   |
10 |   = { IF_SOME {nop} { PUSH int 3 }; };
   |                ^
unexpected 'n'

Actual error message:

morley: ../../contracts/FA1/custom.mtz:9:3:
  |
9 |   test :: '[option int] -> '[int]
  |   ^
unexpected 't'
expecting '}'

This is due to backtracing issues.

Solution: Fixed by narrowing down the try backtracing.

Related issue(s)

Resolves TM-267

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