improved error message when sleeplog timestamp is not in expected format

[vincentvanhees]

Fixes #1184

Checklist before merging:

• [ ] Existing tests still work (check by running the test suite, e.g. from RStudio).
• [ ] Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
• [ ] Clean code has been attempted, e.g. intuitive object names and no code redundancy.
• [ ] Documentation updated:
  • [ ] Function documentation
  • [ ] Chapter vignettes for GitHub IO
  • [ ] Vignettes for CRAN
• [x] Corresponding issue tagged in PR message. If no issue exist, please create an issue and tag it.
• [x] Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
• [ ] Added your name to the contributors lists in the DESCRIPTION file, if you think you made a significant contribution.
• [ ] GGIR parameters were added/removed. If yes, please also complete checklist below.

If NEW GGIR parameter(s) were added then these NEW parameter(s) are:

• [ ] documented in man/GGIR.Rd
• [ ] included with a default in R/load_params.R
• [ ] included with value class check in R/check_params.R
• [ ] included in table of vignettes/GGIRParameters.Rmd with references to the GGIR parts the parameter is used in.
• [ ] mentioned in NEWS.Rd as NEW parameter

If GGIR parameter(s) were deprecated these parameter(s) are:

• [ ] documented as deprecated in man/GGIR.Rd
• [ ] removed from R/load_params.R
• [ ] removed from R/check_params.R
• [ ] removed from table in vignettes/GGIRParameters.Rmd
• [ ] mentioned as deprecated parameter in NEWS.Rd
• [ ] added to the list in R/extract_params.R with deprecated parameters such that these do not produce warnings when found in old config.csv files.