A readable non-export source should refuse by name, not by traceback

Pointed at a path that exists but is not a CMP export — an empty file, or a
SQLite database with a different schema — scripts/prod/import_cmp_learners.py
raises a raw sqlite3.OperationalError instead of the bounded
source-unreadable refusal its siblings give.

Found by the tester on #322 while exercising the new Make targets against a
zero-byte stand-in.

The three CMP importers share a deliberate refusal contract: _readonly(...)
→ _refuse("source-unreadable"), with a named condition code rather than a
traceback (courses/services/cmp_content_import.py:305-312,
accounts/services/cmp_learner_import.py:346-353,
courses/services/cmp_learner_history_import.py:303-310). That contract is why
#322's Make-level guard was scoped to a named operator message rather than to
closing a hole — the importers were already assumed to fail closed on a bad
source.

They do fail closed on an unreadable source. A readable file with the wrong
contents is a different condition, and it surfaces as a stack trace naming a
table, which tells an operator at 2am less than the condition code would.

Severity is low and deliberate to record as such: it fails loudly, writes
nothing, and leaks nothing beyond a table name. It is filed so the refusal
contract is uniform rather than mostly-uniform.

Scope: extend the source check to distinguish "not readable" from "readable but
not a CMP export", with a bounded condition code for the second. Whether the
other two importers need the same treatment should be checked rather than
assumed.

Explicit non-goal: any change to what the importers do with a valid export.

Refs #310, #322

Start in scripts/prod/import_cmp_learners.py and compare its source handling with _readonly(...) and _refuse("source-unreadable") in the three service files named in the issue. Run the importer against an empty file or SQLite database with a different schema, then check the other two importers for the same condition. Done means a readable non-CMP source returns a bounded condition code instead of a raw traceback, without changing valid-export behavior.