Avoid deducing errors as argument instructions after subst

[danakj]

If convert fails after applying a substitution in deduce, fail deduction rather than succeeding deduction with an ErrorInst in the deduced argument.

For example, in toolchain/check/testdata/facet/fail_convert_class_type_to_generic_facet_value.carbon the WrongGenericParam is deduced for the first argument, then substituted into the second parameter, but the argument can not convert to the parameter after substitution. In this case, deduction fails instead of producig a call with an error in the second argument. The resulting semir drops the call with an error argument:

-// CHECK:STDOUT:   %CallGenericMethod.specific_fn: <specific function> = specific_function %CallGenericMethod.ref, @CallGenericMethod(constants.%WrongGenericParam, <error>) [concrete = <error>]
-// CHECK:STDOUT:   %CallGenericMethod.call: init %empty_tuple.type = call %CallGenericMethod.specific_fn() [concrete = <error>]

[jonmeow]

LG