Fix display of exception error messages, #1079

[hdgarrood]

Fixes #1079. Running the test script from that issue now prints

Migrating: ALTER TABLE "person" ALTER COLUMN "age" TYPE INT8
[Debug#SQL] ALTER TABLE "person" ALTER COLUMN "age" TYPE INT8; []
persistent-repro-exe:

Database migration: manual intervention required.
The unsafe actions are prefixed by '***' below:

*** ALTER TABLE "person" DROP COLUMN "age";

This is arguably a breaking change, since if people were catching this exception in their code before, they'll need to change their uses of catch to catch this type. If #969 was previously released without a breaking-level version bump and nobody complained, though, this is probably okay to release without a breaking-level version bump too?

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[parsonsmatt]

A few changes please - also this can be released with 2.11.0.0 which is unreleased so far.

[parsonsmatt]

I've got to update the changelog to make it less confusing - can you put this in 2.11.0.0?

[hdgarrood]

Sure thing! I’ve seen people use a heading “unreleased changes” at the top of changelogs, which seems to work okay.

[parsonsmatt]

I'd rather have a derived Show instance and a custom implementation of displayException.

[hdgarrood]

I tried that to start with, but unfortunately GHC uses Show, not displayException, when exiting due to an uncaught exception 🤷‍♂️ I’m not sure how else to handle it. Maybe the exception type could just be data UnsafeMigrationException = UnsafeMigrationException and we could instead print the human readable error message before throwing it?

[hdgarrood]

That’s not super helpful if people do want to catch the error, though.

[parsonsmatt]

are you serious, GHC? okay, this is fine then

[hdgarrood]

haha yeah that was more or less my reaction too