wal: OpenFromIndex fails if it cannot find previous index

Example:
We save entry 1, 2, 3 to WAL.
If we try to open 100, it should fail.
wal: add ErrFileNotFound and ErrIndexNotFound
Merge pull request #1099 from unihorn/128

wal: OpenFromIndex fails if it cannot find previous index
test: actually test main pkg
etcd: reimplement proxy flag