fix: Ensure that ak._v2.to_json raises errors when appropriate.

[jpivarski]

While looking at to_list, I had some questions for myself about the to_json code right beside it, which seemed to not raise errors on bad JSON. The mistake was in my test: v1 has been silently generating bad JSON, not v2! But anyway, here are explicit tests for to_json error-checking so that this doesn't come up again (for v2 users).

[codecov]

Codecov Report

Merging #1649 (29679bc) into main (9e17f29) will increase coverage by 0.52%.
The diff coverage is 67.67%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/cuda/__init__.py	0.00% <0.00%> (ø)
src/awkward/_v2/_connect/numexpr.py	88.40% <0.00%> (ø)
src/awkward/_v2/_connect/pyarrow.py	88.46% <0.00%> (ø)
...awkward/_v2/_connect/rdataframe/from_rdataframe.py	0.00% <0.00%> (ø)
src/awkward/_v2/_lookup.py	98.68% <ø> (+1.17%)	⬆️
src/awkward/_v2/numba.py	93.47% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_avro_file.py	66.66% <0.00%> (ø)
src/awkward/_v2/operations/ak_local_index.py	100.00% <ø> (ø)
src/awkward/_v2/operations/ak_max.py	87.09% <ø> (ø)
src/awkward/_v2/operations/ak_min.py	87.09% <ø> (ø)
... and 59 more