Process error case for processTextAnnotations

The error of function `processTextAnnotations` was never evaluated.

Add if case to panic in case `processTextAnnotations` has an error.

convenience.go

@@ -162,7 +162,9 @@ func Foreground(color colorful.Color) StyleOption {
 func EnableTextAnnotations() StyleOption {
 	return StyleOption{
 		postProcess: func(s *String, flags map[string]struct{}) {
-			processTextAnnotations(s)
+			if err := processTextAnnotations(s); err != nil {
+				panic(err)
+			}
 		},
 	}
 }

convenience_test.go

@@ -137,6 +137,12 @@ var _ = Describe("convenience functions", func() {
 			}).To(Panic())
 		})
 
+		It("should panic in case a non-existing color", func() {
+			Expect(func() {
+				Style("Foobar{test}", EnableTextAnnotations())
+			}).To(Panic())
+		})
+
 		It("should correctly apply a color to a string that already contains text emphasis", func() {
 			text := Sprintf("text with *bold* and _italic_.")
 			Expect(Style(text, Foreground(Orange))).To(