sw-btrace-to-compiledb: Exclude commands when the input/cwd does not exist.

 * This is particularly useful if the btrace.log build includes autoconf
   tests that invoke the compiler.  Typically, the configure script will
   create a directory and/or file for the test, then delete them after the
   test.
sw-clang-indexer: Add the directory name to the chdir error message.