When Ghostscript reports a DeviceN colorspace with an inappropriate
alternate, the resulting PDF/A may render blank in viewers such as Adobe
Reader (#1187). The error is gated on that Ghostscript warning, which is
the authoritative signal that the *output* is broken.
Previously the error message always told the user to "use
--color-conversion-strategy", which is confusing when they already set
one and it didn't help. Crucially, the warning persists for strategies
that don't actually normalize the colorspace -- notably
UseDeviceIndependentColor (confirmed in #1187) -- so silencing the error
for any non-default strategy would emit a silently-broken PDF/A.
Keep raising whenever Ghostscript still reports the warning, regardless
of strategy, but tailor the guidance: if no conversion was requested,
suggest RGB/CMYK/Gray; if a conversion was requested but the warning
persisted, say so and point at strategies that work or --output-type pdf.
Add unit tests (mocked Ghostscript) covering the default case, the
warning-persists-despite-strategy case for both an ineffective strategy
and a normally-effective one, and the no-warning happy path.
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