Change JsonSemAction to allow non-throw error reporting.

Formerly, semantic action functions for the JSON parser returned void,
so that there was no way for them to affect the parser's behavior.
That means in particular that they can't force an error exit except by
longjmp'ing.  That won't do in the context of our project to make input
functions return errors softly.  Hence, change them to return the same
JsonParseErrorType enum value as the parser itself uses.  If an action
function returns anything besides JSON_SUCCESS, the parse is abandoned
and that error code is returned.

Action functions can thus easily return the same error conditions that
the parser already knows about.  As an escape hatch for expansion, also
invent a code JSON_SEM_ACTION_FAILED that the core parser does not know
the exact meaning of.  When returning this code, an action function
must use some out-of-band mechanism for reporting the error details.

This commit simply makes the API change and causes all the existing
action functions to return JSON_SUCCESS, so that there is no actual
change in behavior here.  This is long enough and boring enough that
it seemed best to commit it separately from the changes that make
real use of the new mechanism.

In passing, remove a duplicate assignment of
transform_string_values_scalar.

Discussion: https://postgr.es/m/1436686.1670701118@sss.pgh.pa.us

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