Saving a draft of a file crashes the watcher

[fhars]

Describe the bug
If you save a draft of a file that does not yet contain all parts, the watcher crashes.

To Reproduce

mkdir test_dir
cd test_dir
cat > foo.md <<'EOF'
``` {.toml file=foo.toml}
<<foo_contents>>
```
EOF
entangled watch

Observed behaviour:

entangled watch terminates with an error message:

[11:26:07] INFO     Welcome to Entangled v2.4.2!
           INFO     Tangling.
           ERROR    foo.md:1: Missing reference `foo_contents`
$

Expected behavior

entagled watch should tangle all files in the directory, print a warning and continue watching.

Bug trace

Desktop (please complete the following information):

• OS: Debian Linux

[jhidding]

I made a fix that catches the error and continue running. In the current architecture it is not so simple to tangle the files that do work. The philosophy being that transactions should be good before execution.