Throw a specific error for incomplete parse errors.

`nix-repl` will use this for deciding whether to keep waiting for input or
error out right away.

src/libexpr/lexer.l

@@ -195,5 +195,7 @@ or          { return OR_KW; }
 
 }
 
+<<EOF>> { data->atEnd = true; return 0; }
+
 %%
 

src/libexpr/nixexpr.hh

@@ -11,6 +11,7 @@ namespace nix {
 
 MakeError(EvalError, Error)
 MakeError(ParseError, Error)
+MakeError(IncompleteParseError, ParseError)
 MakeError(AssertionError, EvalError)
 MakeError(ThrownError, AssertionError)
 MakeError(Abort, EvalError)

src/libexpr/parser.y

@@ -31,10 +31,12 @@ namespace nix {
         Path basePath;
         Symbol path;
         string error;
+        bool atEnd;
         Symbol sLetBody;
         ParseData(EvalState & state)
             : state(state)
             , symbols(state.symbols)
+            , atEnd(false)
             , sLetBody(symbols.create("<let-body>"))
             { };
     };
@@ -539,7 +541,12 @@ Expr * EvalState::parse(const char * text,
     int res = yyparse(scanner, &data);
     yylex_destroy(scanner);
 
-    if (res) throw ParseError(data.error);
+    if (res) {
+      if (data.atEnd)
+        throw IncompleteParseError(data.error);
+      else
+        throw ParseError(data.error);
+    }
 
     data.result->bindVars(staticEnv);