Pipeline generator expects error message field to always be string

[P1llus]

Certain modules has multiple error messages, so the error.message field returned might have more than a single value (an array of multiple values).

The code on this line: d74415f#diff-3a1cbf8695454ceffdb9bbb280ad58c06e82c70cdc8a420700b3804196fc8ce5R338

That was added with #290.

An example would be this PR elastic/integrations#783 (commit f929437), which when running the generator will return:

suricata/eve test-eve-small.log:
[0] can't unmarshal event to check pipeline error: json: cannot unmarshal array into Go struct field .Error.Message of type string

Is it possible to add support for arrays/slices here as well?

[chrisberkhout]

Was fixed in:

• internal/testrunner/runners: don't just say no, say why #714
• internal/testrunner/runners/pipeline: clarify warning of error.message type #1286