Exporter lambdas can loop indefinitely if an error is encountered in a nested lambda

Some strange behaviour is occuring in the spreadsheet exporter lambdas when running into errors.

1. `export-get` is invoked, and within itself, invokes `export-invokable`.
2. `export-invokable` errors out - in the situation that this behaviour was discovered, it was a syntax error, but the behaviour may also occur if `export-invokable` is interrupted.
3. `export-get` loops indefinitely, awaiting a response from `export-invokable` - however, `export-invokable` doesn't appear to be called more than once.

`export-get` appears to have a 15 minute timeout, since exporting the spreadsheet can take time, but we want to ensure it isnt looping itself if it `export-invokable` is interrupted. Looping can rack up high AWS bills very quickly.

This issue should be investigated and a more robust error handling method should be implemented.

Start by tracing the spreadsheet exporter entry points named `export-get` and `export-invokable`, then reproduce the nested-lambda failure using the reported syntax-error or interruption scenario. Done means an error from `export-invokable` is handled without an indefinite wait or repeated billing-causing loop, with coverage for the failure path.

Written by the indexing model from the issue text.