Proto/Michelson: improve the trace for unpackable types

What

Slightly improve error reporting when a value is used in a context where it is expected to be packable (i.e. encoded to be emitted, used as failwith parameter, etc).

Why

This can improve some error messages

How

In script_ir_translator.check_packable, simply add another error to state the function failed due to not being packable.

The backport for proto T will be done in a follow-up MR.

Manually testing the MR

Run a sandbox, and try to originate the following contract:

parameter address;
storage (big_map address nat);
code {
       CDR ;
       PUSH nat 0 ;
       PUSH nat 0 ;
       COMPARE ;
       EQ ;
       IF
         {

         }
         {
            FAILWITH
         };
       DROP ;
       NIL operation ;
       PAIR
     };

Without the new error, it fails with

Ill typed contract:
  01: { parameter address ;
  02:   storage (big_map address nat) ;
  03:   code { CDR ;
  04:          PUSH nat 0 ;
  05:          PUSH nat 0 ;
  06:          COMPARE ;
  07:          EQ ;
  08:          IF {} { FAILWITH } ;
  09:          DROP ;
  10:          NIL operation ;
  11:          PAIR } }
At line 8 characters 17 to 25,
big_map or sapling_state type not expected here

With the new error:

Ill typed contract:
  01: { parameter address ;
  02:   storage (big_map address nat) ;
  03:   code { CDR ;
  04:          PUSH nat 0 ;
  05:          PUSH nat 0 ;
  06:          COMPARE ;
  07:          EQ ;
  08:          IF {} { FAILWITH } ;
  09:          DROP ;
  10:          NIL operation ;
  11:          PAIR } }
At line 8 characters 17 to 25,
Value has a type that cannot be packed
At line 8 characters 17 to 25,
big_map or sapling_state type not expected here

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR