am -3: do not let failed merge from completing the error codepath

am -3: do not let failed merge from completing the error codepath

When "am" was rewritten in C, the codepath for falling back to
three-way merge was mistakenly made to make an internal call to
merge-recursive, disabling the error reporting code for certain
types of errors merge-recursive detects and reports by calling
die().

This is a quick-fix for correctness.  The ideal endgame would be to
replace run_command() in run_fallback_merge_recursive() with a
direct call after making sure that internal call to merge-recursive
does not die().

Signed-off-by: Junio C Hamano <gitster@pobox.com>