block-party: Use a full snafu error rather than an io::Error shim

[tjkirch]

Fixes #307.

Testing done:

growpart grew the part:

bash-5.0# df -h /local
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvdb1       20G  1.4G   18G   8% /local

signpost still sees the things:

bash-5.0# signpost status
OS disk: /dev/xvda
Set A:   boot=/dev/xvda2 root=/dev/xvda3 hash=/dev/xvda4 priority=1 tries_left=0 successful=true
Set B:   boot=/dev/xvda6 root=/dev/xvda7 hash=/dev/xvda8 priority=0 tries_left=0 successful=false
Active:  Set A
Next:    Set A

[tjkirch]

This push removes the unneeded use block_party; statements.