Return Result from ProfileData::gen

Summary: `ProfileData` later in the stack does serialization, not just `String::clone`, so it may fail.

Reviewed By: milend

Differential Revision: D38681026

fbshipit-source-id: d236920c817787c2c3bb1f4d2865f690ba688923

Author: stepancheg

starlark/src/eval/runtime/profile/data.rs

@@ -39,13 +39,13 @@ impl ProfileData {
     }
 
     /// Generate a string with profile data (e.g. CSV or flamegraph, depending on profile type).
-    pub fn gen(&self) -> String {
-        self.profile.clone()
+    pub fn gen(&self) -> anyhow::Result<String> {
+        Ok(self.profile.clone())
     }
 
     /// Write to a file.
     pub fn write(&self, path: &Path) -> anyhow::Result<()> {
-        fs::write(path, &self.profile).with_context(|| {
+        fs::write(path, &self.gen()?).with_context(|| {
             format!(
                 "write profile `{}` data to `{}`",
                 self.profile_mode,