Better error for checkdeps.
Better error for checkdeps.

If the checkdeps script can not be found the following error will be issued
instead of a stacktrace:

** Presubmit ERRORS **
Unable to run checkdeps, does pdfium/buildtools/checkdeps exist?

BUG=chromium:600043

Review URL: https://codereview.chromium.org/1860803003

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium