Make some error messages more consistent with Ruby Sass

Fixes #1941
Spec sass/sass-spec#738

Author: xzyfer

src/parser.cpp

@@ -200,7 +200,7 @@ namespace Sass {
 
     // abort if we are in function context and have nothing parsed yet
     else if (stack.back() == Scope::Function) {
-      error("Functions can only contain variable declarations and control directives", pstate);
+      error("Functions can only contain variable declarations and control directives.", pstate);
     }
 
     // parse imports to process later
@@ -340,7 +340,12 @@ namespace Sass {
   {
     Scope parent = stack.empty() ? Scope::Rules : stack.back();
     if (parent != Scope::Root && parent != Scope::Rules && parent != Scope::Function) {
-      error("Functions may not be defined within control directives or other mixins.", pstate);
+      if (which_type == Definition::FUNCTION) {
+        error("Functions may not be defined within control directives or other mixins.", pstate);
+      } else {
+        error("Mixins may not be defined within control directives or other mixins.", pstate);
+      }
+
     }
     std::string which_str(lexed);
     if (!lex< identifier >()) error("invalid name in " + which_str + " definition", pstate);