Add a clearer error message for InvalidPathError during evaluation

Part of #270, #271

Change-Id: I864d7340f26d3c0f9c45db7b6b545face38d8294

src/libexpr/eval-error.hh

@@ -47,12 +47,16 @@ MakeError(MissingArgumentError, EvalError);
 MakeError(RestrictedPathError, Error);
 MakeError(InfiniteRecursionError, EvalError);
 
+/**
+ * Represents an exception due to an invalid path; that is, it does not exist.
+ * It corresponds to `!Store::validPath()`.
+ */
 struct InvalidPathError : public EvalError
 {
 public:
     Path path;
     InvalidPathError(EvalState & state, const Path & path)
-        : EvalError(state, "path '%s' is not valid", path)
+        : EvalError(state, "path '%s' did not exist in the store during evaluation", path)
     {
     }
 };

src/libstore/store-api.hh

@@ -62,10 +62,18 @@ MakeError(SubstError, Error);
  * denotes a permanent build failure
  */
 MakeError(BuildError, Error);
+/**
+ * denotes that a path in the store did not exist (but it could, had it
+ * been put there, i.e. it is still legal).
+ */
 MakeError(InvalidPath, Error);
 MakeError(Unsupported, Error);
 MakeError(SubstituteGone, Error);
 MakeError(SubstituterDisabled, Error);
+/**
+ * denotes that a path could not possibly be a store path.
+ * e.g. outside of the nix store, illegal characters in the name, etc.
+*/
 MakeError(BadStorePath, Error);
 
 MakeError(InvalidStoreURI, Error);
@@ -328,6 +336,7 @@ public:
 
     /**
      * Check whether a path is valid.
+     * A path is valid when it exists in the store *now*.
      */
     bool isValidPath(const StorePath & path);