Add better error for object literal vs labeled block confusion

A common error is confusing a labeled block for an object literal:

syntax o = ctx => {
  return #`{ a: 1, b: 2 }`
}
o

throws:

Error: not a valid expression : 2

An error is correct since this is not a valid program but this should be a better error message the mentions the common confusion between object literals and labeled blocks.