You have to be sure that the 'Name' tag exists before accessing it. Try this:
import boto.ec2
conn=boto.ec2.connect_to_region("eu-west-1")
reservations = conn.get_all_instances()
for res in reservations:
for inst in res.instances:
if 'Name' in inst.tags:
print "%s (%s) [%s]" % (inst.tags['Name'], inst.id, inst.state)
else:
print "%s [%s]" % (inst.id, inst.state)
will print:
i-4e444444 [stopped]
Amazon Linux (i-4e333333) [running]