Improve error message when report dependencies missing

A number of people have been confused by this, so we might as well add
code to make it more clear

Fixes #500

Author: jimhester

NEWS.md

@@ -1,5 +1,7 @@
 # covr (development version)
 
+* `report()` now provides a more detailed error message if the `DT` and `htmltools` dependencies are not installed (#500).
+
 * Added `covr.record_tests` option. When `TRUE`, this enables the recording of
   the trace of the tests being executed and adds an itemization of which tests
   result in the execution of each trace. For more details see

R/report.R

@@ -21,8 +21,9 @@ report <- function(x = package_coverage(),
   # Paths need to be absolute for save_html to work properly
   file <- file.path(normalizePath(dirname(file), mustWork = TRUE), basename(file))
 
-  loadNamespace("htmltools")
-  loadNamespace("DT")
+  if (!(requireNamespace("htmltools", quietly = TRUE) && requireNamespace("DT", quietly = TRUE))) {
+    stop("The `DT` and `htmltools` packages must be installed to use `covr::report()`", call. = FALSE)
+  }
 
   data <- to_report_data(x)