Use traceback instead of dump.frame

traceback properly ignores errors inside of try blocks, so it is safe to
use as the default error handler.

Fixes #88

DESCRIPTION

@@ -1,6 +1,6 @@
 Package: covr
 Title: Test Coverage for Packages
-Version: 1.1.2
+Version: 1.2.0
 Authors@R: person("Jim", "Hester", email = "[email protected]", role = c("aut", "cre"))
 Description: Track and report code coverage for your package and (optionally)
     upload the results to a coverage service like Codecov (http://codecov.io)

R/subprocess.R

@@ -30,7 +30,7 @@ subprocess <- function(code, calling_env = parent.frame(),
 
   command <- sprintf(
     paste(sep = "\n",
-      "options(error = quote({dump.frames(to.file = TRUE); q(status = 1)}))",
+      "options(error = function() traceback(2))",
       "options(warn = 1)",
       "load('%s')",
       ".code <- readRDS('%s')",