Fix server errors when deleting static files or directories

Fixes #15174

testscripts/server/server__delete_static_file_mount.txt

@@ -0,0 +1,32 @@
+# Test the hugo server command when removing a file mounted into static.
+# See issue 15174.
+
+hugo server &
+
+waitServer
+
+rm myfiles/robots.txt
+
+sleep 2
+
+stopServer
+! stderr .
+
+-- hugo.toml --
+title = "Hugo Server Test"
+baseURL = "https://example.org/"
+disableKinds = ["taxonomy", "term", "sitemap"]
+[[module.mounts]]
+source = "content"
+target = "content"
+[[module.mounts]]
+source = "layouts"
+target = "layouts"
+[[module.mounts]]
+source = "myfiles/robots.txt"
+target = "static/robots.txt"
+-- layouts/all.html --
+All.
+-- content/p1.md --
+-- myfiles/robots.txt --
+robots

testscripts/server/server__delete_static_mount_dir.txt

@@ -0,0 +1,32 @@
+# Test the hugo server command when removing a directory mounted into static.
+# See issue 15174.
+
+hugo server &
+
+waitServer
+
+rm mystatic/foo
+
+sleep 2
+
+stopServer
+! stderr .
+
+-- hugo.toml --
+title = "Hugo Server Test"
+baseURL = "https://example.org/"
+disableKinds = ["taxonomy", "term", "sitemap"]
+[[module.mounts]]
+source = "content"
+target = "content"
+[[module.mounts]]
+source = "layouts"
+target = "layouts"
+[[module.mounts]]
+source = "mystatic/foo"
+target = "static/foo"
+-- layouts/all.html --
+All.
+-- content/p1.md --
+-- mystatic/foo/static.txt --
+static