Update specs for Dart Sass's failed-extend output. (#994)

Author: nex3

spec/errors/extend/placeholder/missing/error-dart-sass

@@ -0,0 +1,5 @@
+Error: The target selector was not found.
+Use "@extend %foo !optional" to avoid this error.
+  @extend %foo;
+  ^^^^^^^^^^^^
+  /sass/spec/errors/extend/placeholder/missing/input.scss 2:3

spec/errors/extend/placeholder/missing/options.yml

@@ -0,0 +1,5 @@
+Error: The target selector was not found.
+Use "@extend .foo !optional" to avoid this error.
+  @extend .foo;
+  ^^^^^^^^^^^^
+  /sass/spec/errors/extend/selector/missing/input.scss 2:3

spec/errors/extend/selector/missing/error-dart-sass

@@ -0,0 +1,5 @@
+Error: The target selector was not found.
+Use "@extend %an-undefined-placeholder !optional" to avoid this error.
+  @extend %an-undefined-placeholder;
+  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  /sass/spec/libsass-issues/issue_1670/input.scss 2:3

spec/errors/extend/selector/missing/options.yml

@@ -1,5 +1,5 @@
 Error: The target selector was not found.
-Use "@extend %foo !optional" to avoid this error.
+Use "@extend .z-depth-half-1 !optional" to avoid this error.
     @extend .z-depth-half-1;
     ^^^^^^^^^^^^^^^^^^^^^^^
   /sass/spec/libsass-issues/issue_1916/input.scss 13:5

spec/libsass-closed-issues/issue_1670/error-dart-sass

@@ -0,0 +1,5 @@
+Error: The target selector was not found.
+Use "@extend .foo !optional" to avoid this error.
+  @extend .foo;
+  ^^^^^^^^^^^^
+  /sass/spec/libsass-issues/issue_871/input.scss 2:3