Allow missing files in sync-files --latest

When running `sync-files` (or `sync-identical-files`) with the `--latest` switch, if one or more of the files in a group does not exist, the script will crash. This happens all the time when I add a new group, or add a new file path in an existing group. This has bothered me for a long time, so I finally fixed it when I ran into it again today.

I've changed the script as follows:
- If _none_ of the paths in the group exist, print an error message listing the paths in the group. This happens with or without `--latest`.
- If `--latest` is specified, copy the master file to the paths of the missing files.

config/sync-files.py

@@ -59,29 +59,41 @@ def file_checksum(filename):
         return hashlib.sha1(file_handle.read()).hexdigest()
 
 def check_group(group_name, files, master_file_picker, emit_error):
-    checksums = {file_checksum(f) for f in files}
+    extant_files = [f for f in files if path.isfile(f)]
+    if len(extant_files) == 0:
+        emit_error(__file__, 0, f"No files found from group '{group_name}'.")
+        emit_error(__file__, 0,
+                "Create one of the following files, and then run this script with "
+                "the --latest switch to sync it to the other file locations.")
+        for filename in files:
+            emit_error(__file__, 0, "    " + filename)
+        return
+
+    checksums = {file_checksum(f) for f in extant_files}
 
-    if len(checksums) == 1:
+    if len(checksums) == 1 and len(extant_files) == len(files):
+        # All files are present and identical.
         return
 
-    master_file = master_file_picker(files)
+    master_file = master_file_picker(extant_files)
     if master_file is None:
         emit_error(__file__, 0,
                 "Files from group '"+ group_name +"' not in sync.")
         emit_error(__file__, 0,
                 "Run this script with a file-name argument among the "
                 "following to overwrite the remaining files with the contents "
-                "of that file or run with the --latest switch to update each "
+                "of that file, or run with the --latest switch to update each "
                 "group of files from the most recently modified file in the group.")
-        for filename in files:
+        for filename in extant_files:
             emit_error(__file__, 0, "    " + filename)
     else:
         print("  Syncing others from", master_file)
         for filename in files:
             if filename == master_file:
                 continue
             print("    " + filename)
-            os.replace(filename, filename + '~')
+            if path.isfile(filename):
+                os.replace(filename, filename + '~')
             shutil.copy(master_file, filename)
         print("  Backups written with '~' appended to file names")