Better error message for update()

Improves the error message for update() when the existing value is the same as the new one, as suggested in #1254
Better error message for update()

Return error messages for both identical values and update failures.
Don't throw error, as this will change existing behavior

More verbose tests
Merge branch 'master' into 1254-same-option-pippin
Merge branch 'master' into 1254-same-option-pippin
Merge pull request #1551 from wp-cli/1254-same-option-pippin

Use a friendly error message when option is same value