cgen, json: make errors more informative (resolve empty panics)

[ttytm]

The changes make error messages for json.decode more informative. The reason is that error messages we'll get when walking back on the cJSON error_ptr can currently be empty and become hard to localize. The example below would currently just print an empty line.

json_txt := 'abc'
_ := json.decode(Foo2, json_txt) or {
	eprintln(err)
}

With the changes the errors message failed to decode JSON string is added. Similar to failed to open file during a os.read_file error.

---

This should help with the first of the two problems initially attempted to solve in #21184.
The second example in the linked PR could be addressed in a separate PR, where cgen is extended to add a check to verify the variable that should be decoded for validity.

_{Huly®: V_0.6-21341}

[spytheman]

I've tried this on master, and you are right - the error is empty :-| ...

import json

struct Foo2 {}

json_txt := 'abc'
x := json.decode(Foo2, json_txt) or { panic(err) }
dump(x)

[spytheman]

Excellent work.