No warning or error is issued when trying to install a non-existent subdir

**Describe the bug**

No warning or error is issued when trying to install a non-existent subdir.

I was using:

```
install_subdir(
join_paths('docs', 'v', meson.project_version()),
install_dir : join_paths(get_option('docdir')),
strip_directory : true
)
```

('v', when I should have been using 'v' +).

Part of the output:

>Installing subdir /home/andy/src/rmw-project/rmw/subprojects/canfigger/docs/v/0.3.0 to /home/andy/src/rmw-project/rmw/subprojects/canfigger/_build/inst_test/usr/local/share/doc/canfigger

**Expected behavior**

I believe meson should error in the case I mentioned above.

**system parameters**

* Native build
* OS: Manjaro 23.1.3
* Python 3.11.6
* Meson 1.3.1
* Ninja 1.11.1

Start by locating the implementation and tests for the install_subdir() Meson function, then reproduce the example with a nonexistent docs/v/0.3.0 path. Done means the missing subdirectory produces the agreed warning or error, with coverage for this case and existing installation behavior preserved.

Written by the indexing model from the issue text.