examples: merge: fixed formatting of error message
examples: merge: less strict header ckecking

This is workaround for some old eblob bug when header flags may get out
of sync.
Also this fixes issues with .index being outdated.
examples: merge: check that open succeeded
examples: merge: prettify usage
examples: merge: make errors more obvoius
Merge pull request #40 from SaveTheRbtz/master

Examples: merge robustness and usability imporvements