Unhandled exception setting a changed meta data value, new key or new value

[pfw]

Describe the bug
When setting a meta value on a dataset there is an unhandled exception if the key is new or the value of an existing value is changed on a key that is non-standard.

To Reproduce
Steps to reproduce the behaviour:

1. Create a repo and import a dataset
2. kart meta set mydataset mykey="myvalue"
3. Will throw an exception but the value is stored.
4. Run step 2 again, no exception as no change
5. `kart meta set mydataset mykey="my new value"
6. Exception again and value stored

Expected behaviour
No exception

Output

Version Info

❯ kart --version
Kart v0.11.5, Copyright (c) Kart Contributors
» GDAL v3.4.0; PROJ v6.3.0
» PyGit2 v1.9.0; Libgit2 v1.4.2; Git v2.33.GIT
» SQLAlchemy v1.4.40; pysqlite3 v2.6.0/v3.31.1; SpatiaLite v5.0.1; Libpq v14.0.5
» SpatialIndex v1.9.3

[AMANISPEACE]

I want to work on this issue. Can you assign me this issue?

[jhunjhunwala1234]

Hi, I would like to work on this bug. it would be great could you please assign this issue to me

[rcoup]

@jhunjhunwala1234 Great! Open a PR when you have something ready.

[Ritam0]

I want to work on this issue please assign me this issue