improved template syntax errors with file, line number and failed token highlighted in error message

Tests/StencilTests/TemplateSpec.swift

@@ -1,5 +1,5 @@
 import Spectre
-import Stencil
+@testable import Stencil
 
 
 func testTemplate() {
@@ -15,5 +15,30 @@ func testTemplate() {
         let result = try template.render([ "name": "Kyle" ])
         try expect(result) == "Hello World"
     }
+
+    $0.it("throws syntax error on invalid for tag syntax") {
+      let template: Template = "Hello {% for name in %}{{ name }}, {% endfor %}!"
+      var error = TemplateSyntaxError("'for' statements should use the following syntax 'for x in y where condition'.")
+      error.token = Token.block(value: "{% for name in %}", at: template.templateString.range(of: "{% for name in %}")!)
+      error = error.contextAwareError(templateName: nil, templateContent: template.templateString)!
+      try expect(try template.render(["names": ["Bob", "Alice"]])).toThrow(error)
+    }
+    
+    $0.it("throws syntax error on missing endfor") {
+      let template: Template = "{% for name in names %}{{ name }}"
+      var error = TemplateSyntaxError("`endfor` was not found.")
+      error.token = Token.block(value: "{% for name in names %}", at: template.templateString.range(of: "{% for name in names %}")!)
+      error = error.contextAwareError(templateName: nil, templateContent: template.templateString)!
+      try expect(try template.render(["names": ["Bob", "Alice"]])).toThrow(error)
+    }
+
+    $0.it("throws syntax error on unknown tag") {
+      let template: Template = "{% for name in names %}{{ name }}{% end %}"
+      var error = TemplateSyntaxError("Unknown template tag 'end'")
+      error.token = Token.block(value: "{% end %}", at: template.templateString.range(of: "{% end %}")!)
+      error = error.contextAwareError(templateName: nil, templateContent: template.templateString)!
+      try expect(try template.render(["names": ["Bob", "Alice"]])).toThrow(error)
+    }
+    
   }
 }