Migrate source CSV dies on long text

This was a bit of confusion I hit initially due to initial naming convention, but if you look at migrate_example_baseball it should help.

  # These are the field names from the source file representing the key
  # uniquely identifying each game - they will be stored in the migration
  # map table as columns sourceid1, sourceid2, and sourceid3.
  identifiers:
    - start_date
    - home_team
    - home_game_number

These are fields that you want to use to track the relationship of source data to imported data - minimally you only need 1 field that uniquely identifies a piece of source content, like a content ID number. Longer fields like body text should not be used to track these relationships.

So, a more meaningful error message should be provided. But in the mean time, use something to hash that really long text and store the hash, since the hash will be short enough.

Re-assigning to new project name-space.

This is really a core migrate support question, nothing specific to CSV migrations.

Duplicate of #2613878: Use hash for Migration source keys, rather than verbatim values