If a .drv cannot be parsed, show its path

Otherwise you just get ‘expected string `Derive(['’ which isn't very helpful.

src/libutil/util.cc

@@ -1041,7 +1041,7 @@ void expect(std::istream & str, const string & s)
     char s2[s.size()];
     str.read(s2, s.size());
     if (string(s2, s.size()) != s)
-        throw Error(format("expected string `%1%'") % s);
+        throw FormatError(format("expected string `%1%'") % s);
 }
 
 

src/libutil/util.hh

@@ -326,6 +326,8 @@ bool hasSuffix(const string & s, const string & suffix);
 /* Read string `s' from stream `str'. */
 void expect(std::istream & str, const string & s);
 
+MakeError(FormatError, Error)
+
 
 /* Read a C-style string from stream `str'. */
 string parseString(std::istream & str);