Nicer error output

Is pretty cryptic output for 'no value was passed':
```
proto_bin.c:135 E> problem while parsing package 'SET' with opaque 8126464
proto_bin.c:136 E> erroneous 'val' section
memcached.c:115 E> Server error 20: Invalid arguments
proto_bin.c:1102 E> memcached error 20: Invalid arguments
```
