Add backtrace information to errors where relevant. (#166)

* Add backtrace information to errors where relevant.

* More backtrace information.

* Add to the FAQ.

candle-core/src/error.rs

@@ -170,6 +170,12 @@ pub enum Error {
 
     #[error(transparent)]
     Wrapped(Box<dyn std::error::Error + Send + Sync>),
+
+    #[error("{inner}\n{backtrace}")]
+    WithBacktrace {
+        inner: Box<Self>,
+        backtrace: Box<std::backtrace::Backtrace>,
+    },
 }
 
 pub type Result<T> = std::result::Result<T, Error>;
@@ -178,4 +184,16 @@ impl Error {
     pub fn wrap(err: impl std::error::Error + Send + Sync + 'static) -> Self {
         Self::Wrapped(Box::new(err))
     }
+
+    pub fn bt(self) -> Self {
+        let backtrace = std::backtrace::Backtrace::capture();
+        match backtrace.status() {
+            std::backtrace::BacktraceStatus::Disabled
+            | std::backtrace::BacktraceStatus::Unsupported => self,
+            _ => Self::WithBacktrace {
+                inner: Box::new(self),
+                backtrace: Box::new(backtrace),
+            },
+        }
+    }
 }