Add FieldError with field name context to parsing errors

[Cvikli]

Summary

When struct parsing fails, errors like MethodError or TypeError are thrown without any indication of which field caused the failure. This makes debugging JSON parsing issues unnecessarily difficult, especially with larger structs.

This PR adds a FieldError exception type that wraps the original error with field context.

Before

# Wrong type
MethodError: Cannot `convert` an object of type Int64 to an object of type String

# Missing required field
TypeError(:typeassert, "", Int64, nothing)

# null for non-nullable
MethodError: Cannot `convert` an object of type Nothing to an object of type String

No indication of which field failed or which struct was being parsed.

After

# Wrong type
FieldError: Bar.path::String: MethodError: Cannot `convert`...

# Missing required field
FieldError: Bar.count::Int64: ArgumentError: required field is missing from input

# null for non-nullable
FieldError: Bar.path::String: MethodError: Cannot `convert`...

# Nested struct failure
FieldError: Outer.inner::Inner: ArgumentError: applyeach not applicable...

Implementation

• FieldError <: Exception — exported, contains struct_type, field_name, field_type, and error fields for programmatic access
• _make_field helper — wraps the make() + setval!() calls in findfield() with a try-catch that adds field context
• Pre-construction check in makestruct — detects missing required fields before _construct is called, giving a clear error instead of a cryptic TypeError
• Already-wrapped FieldErrors from nested structs are rethrown as-is (no double-wrapping)

Test plan

• All 165 existing tests pass
• Tested with JSON.jl v1.0 against all failure modes: wrong type, missing field, null→non-nullable, empty object, nested struct failure, array of structs
• Valid parsing still works (extra fields ignored, defaults applied)

[quinnj]

Hmmmmm............this looks like the right direction. I'm mostly worried about performance implications here. This code can be super performance-sensitive because it powers, for example, the raw JSON.parse performance. And here we're adding a try-catch to every field parsed, possibly non-inlined new function call, and a for-loop to check every field for every struct. In the JSON.jl repo, we have some benchmarks I worked with to track the performance of things as we iterated and refactored things. Will you report here the implications of these changes across a few of those benchmark cases? Let's see what the impact is and then maybe decide if there are more performant ways to achieve the same helpful behavior.

[quinnj]

Closing because this implementation is no longer release-safe after #65.

The diagnostic goal is still useful, but this patch changes the old findfield hot path, adds an extra required-field scan, exports a new API, and includes no package regression tests. The requested performance comparison was also not provided. The new field-sink path needs a fresh design that adds field context without slowing successful parsing, with direct tests and JSON benchmarks. A focused follow-up PR is welcome.