Fixes r-lib#770

I tried to make the error messages extremely verbose by passing in information about the file name and section tag from where the error originated. This may be a little fragile, but it didn't break any existing tests.

- Added unit test for as_html.tag_url
- Made errors more verbose by changing stopifnot() to if-then with a descriptive stop message in multiple cases (e.g., as_html.tag_url, as_html.tag_eqn, etc)
- Also trying to pass in information on the Section and File from which the error was thrown.

passes all tests in  testthat::test_file("tests/testthat/test-rd-html.R")

- Combines similar code into one function
- Passes all 67 tests in testthat::test_file("tests/testthat/test-rd-html.R")
  - Note that the test uses a call to rlang::bind_env(), which helped me to make sure the function worked as desired. This may however have some side effects that I dont know about.