ExternalData: Add backtrace to missing file warning

Since commit 32e48028 (If the original file is missing, make it
non-fatal, 2013-10-31) we use a STATUS message instead of a FATAL_ERROR
to report that a DATA{} reference names a missing file.  The STATUS
message has no backtrace.  Use an AUTHOR_WARNING message instead.

Also restore the FATAL_ERROR message when the DATA{} reference names a
file that exists as a directory.

Change-Id: I2cbcb8f6667074401ea9f7743cf6e547ec5061a0
Merge topic 'ExternalData-missing-data-backtrace' into master

94f12816 ExternalData: Add backtrace to missing file warning