Change the .text-too-small error message.

The original error message apparently confuses people, and this
can be easily improved.  See the following:
https://community.rapid7.com/thread/2356

lib/msf/util/exe.rb

@@ -255,8 +255,11 @@ require 'digest/sha1'
 			raise RuntimeError, "The .text section does not contain an entry point"
 		end
 
-		if(text.size < (payload.length + 256))
-			raise RuntimeError, "The .text section is too small to be usable"
+		p_length = payload.length + 256
+		if(text.size < p_length)
+			msg  = "The .text section is too small. "
+			msg << "Minimum is #{p_length.to_s} bytes, your .text section is #{text.size.to_s} bytes"
+			raise RuntimeError, msg
 		end
 
 		# Store some useful offsets