Fix #170, improve cell attribute error messages (#171)

src/server.jl

@@ -953,7 +953,19 @@ function extract_cell_options(source::AbstractString; file::AbstractString, line
                   """
             error(msg)
         end
-        isa(options, Dict) || error("Cell attributes must be a dictionary.")
+        if !isa(options, Dict)
+            msg = """
+                  Invalid cell attributes type at $(file):$(line):
+
+                  ```{julia}
+                  $source
+                  ```
+
+                  Expected a dictionary, got $(typeof(options)). Check for
+                  syntax errors in the YAML block at the start of this cell.
+                  """
+            error(msg)
+        end
         return options
     end
 end

test/testsets/cell_options.jl

@@ -53,6 +53,17 @@ end
                 line = 1,
             )
 
+            text = """
+                   ```{julia}
+                   #| invalid:true
+                   ```
+                   """
+            @test_throws_message "file.qmd:1" QuartoNotebookRunner.extract_cell_options(
+                text;
+                file = "file.qmd",
+                line = 1,
+            )
+
             text = """
                    ```{julia}
                    a = 1
@@ -81,7 +92,7 @@ end
             server = QuartoNotebookRunner.Server()
 
             buffer = IOBuffer()
-            @test_throws_message "Cell attributes must be a dictionary." QuartoNotebookRunner.run!(
+            @test_throws_message "Invalid cell attributes type at" QuartoNotebookRunner.run!(
                 server,
                 notebook;
                 output = buffer,