Better error message

[6twenty]

Ensures that the error raised contains the from and to currency codes.

Prior to this change, the error raised would not contain the from & to currency codes, eg:
Exchange::NoRateError: No rates where found for to 1.380024 at Sun Aug 01 00:00:00 +1200 2010

This is because the method only raises an error when from_code and to_code are nil. This is an perfectly fine way to raise the error, but it also means that those nil values are getting used in the error message string. This commit supplies the original from & to codes as well, so they they may be used in the error message.

This is a fairly ugly and direct way to fix this issue, so feel free to take it or leave it -- but please do add the currency codes to the error message one way or another.

[julianbei]

+1

[beatrichartz]

I will put some work into this in the next days in order to provide a patch. Thanks for raising the issue.