Improved implementation of ExecutionPoint in order to get better test…

… error messages when no backtrace is supplied or an empty backtrace is supplied.

test/execution_point.rb

@@ -11,10 +11,12 @@ def initialize(backtrace)
   end
 
   def file_name
+    return "unknown" unless @backtrace && @backtrace.first
     /\A(.*?):\d+/.match(@backtrace.first)[1]
   end
 
   def line_number
+    return "unknown" unless @backtrace && @backtrace.first
     Integer(/\A.*?:(\d+)/.match(@backtrace.first)[1])
   end
 
@@ -24,7 +26,7 @@ def ==(other)
   end
 
   def to_s
-    "file: #{file_name} line: #{line_number}"
+    "file: #{file_name}; line: #{line_number}"
   end
 
   def inspect