feat: Show nicer error if cachedir can't be created

If we cannot create the cache directory to install the elk.js
dependencies, we used to crash with whatever error message the OS gave
us. This can sometimes be misleading, e.g. when the message is talking
about missing permissions, but in reality `$HOME` or `$XDG_CACHE_HOME`
is simply set to a broken value.

This commit catches an OSError here and instead shows a nicer error
message (and also much more concise stack trace) with a concrete hint
towards `$HOME`.

This is mostly relevant when running inside a container or as a system
service, as it's much easier to accidentally miss some envvars there.

capellambse_context_diagrams/_elkjs.py

@@ -307,8 +307,15 @@ def _install_npm_package(npm_pkg_name: str, npm_pkg_version: str) -> None:
 
 
 def _install_required_npm_pkg_versions() -> None:
-    if not NODE_HOME.is_dir():
-        NODE_HOME.mkdir(parents=True)
+    try:
+        NODE_HOME.mkdir(parents=True, exist_ok=True)
+    except OSError as err:
+        raise RuntimeError(
+            f"Cannot create elk.js install directory at: {NODE_HOME}.\n"
+            "Make sure that important environment variables"
+            " like $HOME are set correctly.\n"
+            f"Failed due to {type(err).__name__}: {err}"
+        ) from None
     installed = _get_installed_npm_pkg_versions()
     for pkg_name, pkg_version in REQUIRED_NPM_PKG_VERSIONS.items():
         if installed.get(pkg_name) != pkg_version: