treewide: Add Store::requireStoreObjectAccessor, simplify uses of getFSAccessor

This is a simple wrapper around getFSAccessor that throws an InvalidPath
error. This simplifies usage in callsites that only care about getting
a non-null accessor.

src/libstore/include/nix/store/store-api.hh

@@ -724,10 +724,28 @@ public:
      * the Nix store.
      *
      * @return nullptr if the store doesn't contain an object at the
-     * givine path.
+     * given path.
      */
     virtual std::shared_ptr<SourceAccessor> getFSAccessor(const StorePath & path, bool requireValidPath = true) = 0;
 
+    /**
+     * Get an accessor for the store object or throw an Error if it's invalid or
+     * doesn't exist.
+     *
+     * @throws InvalidPath if the store object doesn't exist or (if requireValidPath = true) is
+     * invalid.
+     */
+    [[nodiscard]] ref<SourceAccessor> requireStoreObjectAccessor(const StorePath & path, bool requireValidPath = true)
+    {
+        auto accessor = getFSAccessor(path, requireValidPath);
+        if (!accessor) {
+            throw InvalidPath(
+                requireValidPath ? "path '%1%' is not a valid store path" : "store path '%1%' does not exist",
+                printStorePath(path));
+        }
+        return ref<SourceAccessor>{accessor};
+    }
+
     /**
      * Repair the contents of the given path by redownloading it using
      * a substituter (if available).

src/libstore/store-api.cc

@@ -1130,7 +1130,7 @@ Derivation Store::derivationFromPath(const StorePath & drvPath)
 
 static Derivation readDerivationCommon(Store & store, const StorePath & drvPath, bool requireValidPath)
 {
-    auto accessor = store.getFSAccessor(drvPath, requireValidPath);
+    auto accessor = store.requireStoreObjectAccessor(drvPath, requireValidPath);
     try {
         return parseDerivation(store, accessor->readFile(CanonPath::root), Derivation::nameFromPath(drvPath));
     } catch (FormatError & e) {