Fix --string non-string error reporting

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Motivation

--string mode leaked Internal error: upickle.core.Abort when the Jsonnet program produced a non-string result (object, array, number, boolean, null). Other implementations report a clean type error. The error path also lost the source position, making debugging harder.

Implementation	--string on {}
C++ jsonnet 0.22.0	ERROR: expected string result, got: object
go-jsonnet 0.22.0	expected string result, got: object
sjsonnet master	Internal error: upickle.core.Abort
this PR	expected string result, got: object

Modification

• Replace the generic SimpleVisitor that threw upickle.core.Abort with a typed visitor that overrides every non-string visitor method to throw sjsonnet.Error with a clear message including the Jsonnet type name (object, array, number, boolean, null).
• Remove unnecessary return keyword in ManifestModule.scala match expression for idiomatic Scala.
• Add a golden test for each non-string type and tighten existing --exec --string tests.

Result

• Non-string results produce a clean sjsonnet.Error with position and Jsonnet type name.
• String happy path is unchanged — zero overhead on the common case.
• Error-path only: fillInStackTrace override prevents wasted JVM stack capture.

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Superseded by combined CLI PR #1032