workspace: fix error while purging all build directories

Author: pietroalbini

CHANGELOG.md

@@ -24,6 +24,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `cargo-install-upgrade` crate. This will speed up the workspace
   initialization.
 
+### Fixed
+
+- Calling `Workspace::purge_all_build_dirs` returned an error when no
+  directories were present instead of doing nothing.
+
 ## [0.2.0] - 2019-09-06
 
 ### Added

src/workspace.rs

@@ -171,7 +171,10 @@ impl Workspace {
 
     /// Remove all the contents of all the build directories, freeing disk space.
     pub fn purge_all_build_dirs(&self) -> Result<(), Error> {
-        remove_dir_all(self.builds_dir())?;
+        let dir = self.builds_dir();
+        if dir.exists() {
+            remove_dir_all(&dir)?;
+        }
         Ok(())
     }