Error for unknown blocks

Stencil/Parser.swift

@@ -74,8 +74,12 @@ public class TokenParser {
             return nodes
         }
 
-        if let tag = tag, let parser = self.tags[tag] {
+        if let tag = tag {
+          if let parser = self.tags[tag] {
             nodes.append(try parser(self, token))
+          } else {
+            throw TemplateSyntaxError("Unknown template tag '\(tag)'")
+          }
         }
       case .Comment:
         continue

StencilSpecs/ParserSpec.swift

@@ -44,4 +44,12 @@ describe("TokenParser") {
     let nodes = try parser.parse()
     try expect(nodes.count) == 1
   }
+
+  $0.it("errors when parsing an unknown tag") {
+    let parser = TokenParser(tokens: [
+      Token.Block(value: "unknown"),
+    ])
+
+    try expect(try parser.parse()).toThrow(TemplateSyntaxError("Unknown template tag 'unknown'"))
+  }
 }