Guard from more non-existent directories errors (#79)

library/Fs.re

@@ -35,4 +35,10 @@ let writeFile = (path, contents) => {
   Lwt.return();
 };
 
+let exists = path => {
+  try%lwt (Lwt_unix.file_exists(path)) {
+  | Unix.Unix_error(_, _, _) => Lwt.return(false)
+  };
+};
+
 let realpath = Filename.realpath;

library/Versions.re

@@ -36,9 +36,13 @@ module Local = {
   let getLatestInstalledNameByPrefix = prefix => {
     open Lwt;
     let%lwt versions =
+      Lwt.catch(
+        () =>
           Fs.readdir(Directories.nodeVersions)
           >|= List.filter(isVersionFitsPrefix(prefix))
-      >|= List.sort(flip(compare));
+          >|= List.sort(flip(compare)),
+        _ => Lwt.return_nil,
+      );
     switch (versions) {
     | [version, ...xs] => Lwt.return_some(version)
     | [] => Lwt.return_none
@@ -291,8 +295,8 @@ let parse = version => {
   let aliasPath = Aliases.toDirectory(version);
   let versionPath = Local.toDirectory(formattedVersion);
 
-  let%lwt aliasExists = Lwt_unix.file_exists(aliasPath)
+  let%lwt aliasExists = Fs.exists(aliasPath)
-  and versionExists = Lwt_unix.file_exists(versionPath)
+  and versionExists = Fs.exists(versionPath)
   and versionByPrefixPath =
     Local.getLatestInstalledNameByPrefix(formattedVersion);