Fix fit$time() for the case when chains fail

[rok-cesnovar]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Fixes #655

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Rok Češnovar

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

Merging #671 (ca0eec1) into master (ee939e7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #671   +/-   ##
=======================================
  Coverage   92.06%   92.07%           
=======================================
  Files          12       12           
  Lines        3530     3531    +1     
=======================================
+ Hits         3250     3251    +1     
  Misses        280      280           

Impacted Files	Coverage Δ
R/run.R	95.98% <100.00%> (+<0.01%)	⬆️

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