Fixed missing optional cases as well as creating a common trait
for readMessage[?] usage.
fix error cases for -I and --proto_path

if you use "-I(folder that doesn't exist yet)", scalabuff will exit silently. this fixes the missing "else" to make it print a warning and continue. i also fixed the same bug in --proto_path while i was in there.
Merge pull request #48 from robey/patch-1

fix error cases for -I and --proto_path
Merge pull request #47 from cideas/extendable-fix

Fixed missing optional cases as well as creating a common trait