[Plasma] Improve flushing error message

[asfimport]

Including the total amount of space taken up in the store helps us diagnose the flushing policy better.

Reporter: Philipp Moritz / @pcmoritz
Assignee: Philipp Moritz / @pcmoritz

PRs and other links:

• GitHub Pull Request #2458

_{Note: This issue was originally created as ARROW-3105. Please see the migration documentation for further details.}

[asfimport]

Philipp Moritz / @pcmoritz:
Issue resolved by pull request 2458
#2458