str.format() doesn't expect a mapping object. Try this:
from collections import defaultdict
d = defaultdict(str)
d['error2'] = "success"
s = "i am an {0[error]} example string {0[error2]}"
print s.format(d)
You make a defaultdict with a str() factory that returns "". Then you make one key for the defaultdict. In the format string, you access keys of the first object passed. This has the advantage of allowing you to pass other keys and values, as long as your defaultdict is the first argument to format().
Also, see http://bugs.python.org/issue6081
