add runtime error

src/mechanize.cr

@@ -2,6 +2,7 @@ require "./mechanize/http/agent"
 require "./mechanize/form"
 require "./mechanize/node"
 require "./mechanize/page"
+require "./mechanize/errors/*"
 
 class Mechanize
   VERSION = "0.1.0"

src/mechanize/errors/base_error.cr

@@ -0,0 +1,3 @@
+# Base Error class
+class MechanizeCr::Error < RuntimeError
+end

src/mechanize/errors/element_not_found_error.cr

@@ -0,0 +1,13 @@
+require "./base_error"
+class MechanizeCr::ElementNotFoundError < MechanizeCr::Error
+  getter element : Symbol
+  getter conditions : String
+
+  def initialize(element, conditions)
+    @element    = element 
+    @conditions = conditions
+
+    super "Element #{element} with conditions #{conditions} was not found"
+  end
+  
+end

src/mechanize/form.cr

@@ -72,15 +72,17 @@ class MechanizeCr::Form
       # TODO
       # when args whose type isn't String is given
     end
-    fields.select do |field|
+    f = fields.select do |field|
       value.all? do |k,v|
         v === field.name
       end
     end
+    f.empty? ? nil : f
   end
 
   def field_with(criteria)
     f = fields_with(criteria)
-    f.empty? ? Array(MechanizeCr::FormContent::Field).new : f.first
+    raise MechanizeCr::ElementNotFoundError.new(:field, criteria) if f.nil?
+    f.first
   end
 end