add argument validation to job::execute and job::repeat statements

Tried [contracts.ruby](https://github.com/egonSchiele/contracts.ruby)
but does not allow mixing of default values and blocks
[contracts.ruby/214](egonSchiele/contracts.ruby#214).
Additionally, contracts.ruby gives developer friendly messages but does
not give user friendly messages.

Instead, rolled by own argument validation.  Error messages include
expected type, actual value, actual type, and source location.

Addresses part of issue #5.  Need validation on remaining DSL statements
to close.

Author: rfdonnelly

lib/jobrnr/dsl/job_builder.rb

@@ -6,6 +6,14 @@ def initialize(id:, predecessors:)
       end
 
       def execute(command = nil, &block)
+        if command.nil? && block.nil?
+          raise Jobrnr::ArgumentError, "'execute' expects a String or a block @ #{source}"
+        elsif !command.nil? && !block.nil?
+          raise Jobrnr::ArgumentError, "'execute' expects a String or block not both @ #{source}"
+        elsif !command.nil? && !command.is_a?(String)
+          raise Jobrnr::ArgumentError, "'execute' expects a String or a block but was given value of '#{command}' of type '#{command.class.name}' @ #{source}"
+        end
+
         if command.nil?
           @obj.command = block
         else
@@ -14,9 +22,17 @@ def execute(command = nil, &block)
       end
 
       def repeat(times)
+        if !times.is_a?(Integer) || times < 0
+          raise Jobrnr::TypeError, "'repeat' expects positive Integer value but was given value of '#{times}' of type '#{times.class.name}' @ #{source}" 
+        end
+
         @obj.iterations = times
       end
 
+      def source
+        caller[2].split(/:/)[0..1].join(':')
+      end
+
       def build
         @obj
       end

lib/jobrnr/error.rb

@@ -4,4 +4,5 @@ module Jobrnr
   class Error < StandardError; end
   class OptionsError < Error; end
   class ImportError < Error; end
+  class ArgumentError < Error; end
 end