fix: always emit a message from emitError

[nicholasgriffintn]

Resolves #564

Description:

As described in the issue, #560 made a change that meant that all emitted events always included metadata as the last parameter, this meant that message was no longer the second parameter in error events which breaks some handling for users who where using that method to get the message details.

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Why is this change required?:

This ensures that the change is backwards compatible by making emitError emit undefined as the second parameter if messages were not provided to it.

NOTE: This does changed the type from setting void in the return union for the error emitted to event to be more explicitly undefined

[github-actions]

LCOV of commit 0883347 during Comment PR Coverage #114

Summary coverage rate:
  lines......: 97.3% (976 of 1003 lines)
  functions..: 97.3% (36 of 37 functions)
  branches...: no data found

Files changed coverage rate:
                   |Lines       |Functions  |Branches    
  Filename         |Rate     Num|Rate    Num|Rate     Num
  =======================================================
  src/consumer.ts  | 3.2%    656| 0.0%    21|    -      0