Improve column_count error handling and documentation (#541)

Closes #261

Author: mlafeldt

crates/duckdb/src/column.rs

@@ -49,6 +49,9 @@ impl Statement<'_> {
     /// If associated DB schema can be altered concurrently, you should make
     /// sure that current statement has already been stepped once before
     /// calling this method.
+    ///
+    /// # Caveats
+    /// Panics if the query has not been [`execute`](Statement::execute)d yet.
     #[inline]
     pub fn column_count(&self) -> usize {
         self.stmt.column_count()

crates/duckdb/src/raw_statement.rs

@@ -71,7 +71,7 @@ impl RawStatement {
 
     #[inline]
     pub fn result_unwrap(&self) -> ffi::duckdb_arrow {
-        self.result.unwrap()
+        self.result.expect("The statement was not executed yet")
     }
 
     #[inline]
@@ -198,6 +198,8 @@ impl RawStatement {
         }
     }
 
+    // FIXME(mlafeldt): This currently panics if the query has not been executed yet.
+    // The C API doesn't have a function to get the column count without executing the query first.
     #[inline]
     pub fn column_count(&self) -> usize {
         unsafe { ffi::duckdb_arrow_column_count(self.result_unwrap()) as usize }