Roles/Keysum: close #130: generate keysum with name + size
Error: close #131: added `is_error` attribute

- When an `Error` object is passed back to the caller of a method, the
  caller can test for `is_error`, and if true, handle the `Error`
  object appropriately
WADIndexer: close #128; return Error objects for all file I/O

- Create the WADFile object near the top of `index_wad`, before the
  calls to `open()/seek()/read()`, so that the checksums/keysum can be
  used earlier in the log output
WADIndex: added check for when $dir_offset > $wadfile->size

- Reworked the error message for reading in the WAD header
wadindex: issue #128; handle errors returned from WADIndexer

- Issue #131; reformat checking for Error objects
  ($obj->can(q(is_error))
- Log 'info' checksums for WAD files
- Reformatted the log "dividers", log statements between major blocks
  of code, in order to make the log output easier to follow