Add the ERRORS doc section.

README.rst

@@ -460,7 +460,24 @@ log; to see it, set the ``varnishd`` parameter ``vsl_mask`` to
 ERRORS
 ======
 
-XXX ...
+If errors are encountered during compression or decompression, they
+are reported in the Varnish log with the tag ``FetchError`` (see
+``vsl(7)``). The ``FetchError`` message for decompression errors
+includes the error message from the brotli decoder library.
+
+If the decoder reaches end-of-stream indicators, signaling a completed
+decompression, but there are still additional bytes in the response
+body, then the filter fails with the ``FetchError`` message "Junk
+after brotli data".
+
+The backend fetch fails when any VFP filter fails, which can lead to a
+"503 Backend fetch failed" response from Varnish. When streaming is
+enabled (i.e when the VCL variable ``beresp.do_stream`` variable is
+``true``, which is the default), the error might not be detected until
+after the response code and header have already been sent to the
+client; then it is too late to change the response status. In such
+cases, the client response body may be empty, and the network
+connection to the client is closed.
 
 REQUIREMENTS
 ============

src/vfp_brotli.vcc

@@ -424,7 +424,24 @@ log; to see it, set the ``varnishd`` parameter ``vsl_mask`` to
 ERRORS
 ======
 
-XXX ...
+If errors are encountered during compression or decompression, they
+are reported in the Varnish log with the tag ``FetchError`` (see
+``vsl(7)``). The ``FetchError`` message for decompression errors
+includes the error message from the brotli decoder library.
+
+If the decoder reaches end-of-stream indicators, signaling a completed
+decompression, but there are still additional bytes in the response
+body, then the filter fails with the ``FetchError`` message "Junk
+after brotli data".
+
+The backend fetch fails when any VFP filter fails, which can lead to a
+"503 Backend fetch failed" response from Varnish. When streaming is
+enabled (i.e when the VCL variable ``beresp.do_stream`` variable is
+``true``, which is the default), the error might not be detected until
+after the response code and header have already been sent to the
+client; then it is too late to change the response status. In such
+cases, the client response body may be empty, and the network
+connection to the client is closed.
 
 REQUIREMENTS
 ============