Use error measure fns from `evalcast`

`Report/error_measures.R` redefines (looks like copy-pasted, but verify) error measures from `evalcast`. We want them to match exactly and also use new efficient versions. Error functions are used in `Report/create_reports.R`.

https://github.com/cmu-delphi/forecast-eval/pull/245 copy-pastes the efficient versions from https://github.com/cmu-delphi/covidcast/pull/602. Once https://github.com/cmu-delphi/covidcast/pull/602 is merged, pull from `evalcast` instead.

Start with Report/error_measures.R and Report/create_reports.R to identify the local definitions and their call sites. Review the referenced forecast-eval and covidcast pull requests, then check evalcast after covidcast pull request 602 is merged. Done means the reports use evalcast's matching efficient error-measure functions without the duplicated local implementations.

Written by the indexing model from the issue text.