Uses anyhow in accounts-hash-cache-tool

[brooksprumo]

Problem

The agave-hash-cache-tool provides displays messages on errors. This could be even more helpful by using anyhow.

Additionally, this reduces developer burden to manually handle displaying error messages correctly.

Summary of Changes

Use anyhow::Result<_> instead of std::result::Result<_, String>.

Here's an example of the output:

$ agave-accounts-hash-cache-tool diff files bad-file1 bad-file2
Error: 'diff files' failed

Caused by:
    0: failed to extract entries from file 1
    1: failed to open accounts hash cache file 'bad-file1'
    2: No such file or directory (os error 2)

[HaoranYi]

lgtm.