Store reason as a field in MissingExperimentalFeature

Store the reason string as a field in the exception class rather than
only embedding it in the error message. This supports better structured
error handling and future JSON error reporting.

Suggested by Ericson2314 in PR review.

src/libutil/experimental-features.cc

@@ -383,6 +383,7 @@ MissingExperimentalFeature::MissingExperimentalFeature(ExperimentalFeature featu
           showExperimentalFeature(feature),
           Uncolored(optionalBracket(" (", reason, ")")))
     , missingFeature(feature)
+    , reason{reason}
 {
 }
 

src/libutil/include/nix/util/experimental-features.hh

@@ -88,6 +88,8 @@ public:
      */
     ExperimentalFeature missingFeature;
 
+    std::string reason;
+
     MissingExperimentalFeature(ExperimentalFeature missingFeature, std::string reason = "");
 };