fix(samplers): honor fallback setting in remaining sampler fallback paths

create_sampler restored the model default scheduler on a prediction-type
mismatch, on an unknown sampler config, and on any scheduler-constructor
exception regardless of schedulers_fallback; only SD_SAMPLER_DEBUG could turn
the prediction mismatch into an error. Raise like the other capability gates
when the fallback setting is disabled.

An unresolved sampler name substituted UniPC before any of those gates could
run; pass the requested name through instead, so it falls back to the model
default (or raises when fallback is disabled) and the infotext records
Default rather than the unresolved name. find_sampler now also resolves an
unspecified sampler to Default instead of UniPC, matching the platform
default used everywhere else.

modules/sd_samplers_diffusers.py

@@ -519,6 +519,8 @@ class DiffusionSampler:
             log.error(f'Sampler: "{name}" {e}')
             if debug:
                 errors.display(e, 'Samplers')
+            if debug or not shared.opts.schedulers_fallback:
+                raise
             self.sampler = None
             return