rustc: Use spans for #env errors

Issue #444

src/test/compile-fail/extenv-no-args.rs

@@ -0,0 +1,5 @@
+// error-pattern:malformed #env call
+
+fn main() {
+  #env();
+}

src/test/compile-fail/extenv-not-string-literal.rs

@@ -0,0 +1,5 @@
+// error-pattern:malformed #env call
+
+fn main() {
+  #env(10);
+}

src/test/compile-fail/extenv-too-many-args.rs

@@ -0,0 +1,5 @@
+// error-pattern:malformed #env call
+
+fn main() {
+  #env("one", "two");
+}