diff --git a/Dockerfile b/Dockerfile
index 665ee8b..9b8527e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -11,6 +11,8 @@ RUN \
   apt-get update && \
   apt-get install -y --no-install-recommends python3-fitz libreoffice-nogui libreoffice-java-common python3 python3-magic default-jdk-headless fonts-noto-cjk && \
   : "Clean up for improving reproducibility (optional)" && \
+  rm -rf /var/cache/fontconfig/ && \
+  rm -rf /etc/ssl/certs/java/cacerts && \
   rm -rf /var/log/* /var/cache/ldconfig/aux-cache /var/lib/apt/lists/*
 
 RUN mkdir -p /opt/dangerzone/dangerzone && \
@@ -18,7 +20,7 @@ RUN mkdir -p /opt/dangerzone/dangerzone && \
   addgroup --gid 1000 dangerzone && \
   adduser --uid 1000 --ingroup dangerzone --shell /bin/true --home /home/dangerzone dangerzone
 
-COPY conversion /opt/dangerzone/dangerzone/conversion
+COPY conversion/doc_to_pixels.py conversion/common.py conversion/errors.py conversion/__init__.py /opt/dangerzone/dangerzone/conversion
 
 ###########################################
 # gVisor wrapper image
diff --git a/dangerzone/conversion/common.py b/dangerzone/conversion/common.py
index 0940a56..2ff10ec 100644
--- a/dangerzone/conversion/common.py
+++ b/dangerzone/conversion/common.py
@@ -8,6 +8,15 @@ DEFAULT_DPI = 150  # Pixels per inch
 INT_BYTES = 2
 
 
+class CommandError(RuntimeError):
+
+    def __init__(self, msg, stdout, stderr):
+        self.stdout = stdout
+        self.stderr = stderr
+        msg += f"\n====\nCommand output:\n{stdout}\n=====\nCommand stderr:\n{stderr}\n======"
+        super().__init__(msg)
+
+
 def running_on_qubes() -> bool:
     # https://www.qubes-os.org/faq/#what-is-the-canonical-way-to-detect-qubes-vm
     return os.path.exists("/usr/share/qubes/marker-vm")
@@ -96,7 +105,7 @@ class DangerzoneConverter:
         Run a command using asyncio.subprocess, consume its standard streams, and return its
         output in bytes.
 
-        :raises RuntimeError: if the process returns a non-zero exit status
+        :raises CommandError: if the process returns a non-zero exit status
         """
         # Start the provided command, and return a handle. The command will run in the
         # background.
@@ -125,13 +134,15 @@ class DangerzoneConverter:
         # Wait until the command has finished. Then, verify that the command
         # has completed successfully. In any other case, raise an exception.
         ret = await proc.wait()
-        if ret != 0:
-            raise RuntimeError(error_message)
 
         # Wait until the tasks that consume the command's standard streams have exited as
         # well, and return their output.
         stdout = await stdout_task
         stderr = await stderr_task
+
+        if ret != 0:
+            raise CommandError(error_message, stdout, stderr)
+
         return (stdout, stderr)
 
     @abstractmethod