Recommend an alternate action when skyline optimization fails
Recommend an alternate action when skyline optimization fails

The refine command can fail when skyline optimization in TreeTime fails.
These failures are most likely data-dependent, but this commit updates
the error message for this edge case to let the user know they should
try another approach.

Related to #709

blab/nextstrain-augur,nextstrain/augur,nextstrain/augur,nextstrain/augur