Ensure correct compiler error styling and strip ANSI escape sequences.

The compiler error output that's injected into the page hard-codes a
background color of white, but didn't hard-code a corresponding text
color. If the page had set a default text color that was too close to
white, it would be hard or impossible to read without selecting the text
on the page.

This change addresses this by hard-coding a text color of black,
ensuring it can be read.

It also goes a step further and improves the display when dealing with
ANSI escape sequences in the error output, making it difficult to read
through or copy/paste meaningful results. We now apply a common regex
for stripping away any typical ANSI escape sequences we should expect in
such output, leaving behind only the plain text content.

pipeline/templates/pipeline/compile_error.html

@@ -1,5 +1,5 @@
 <div id="django-pipeline-error-{{package_name}}" class="django-pipeline-error"
-     style="display: none; border: 2px #DD0000 solid; margin: 1em; padding: 1em; background: white;">
+     style="display: none; border: 2px #DD0000 solid; margin: 1em; padding: 1em; background: white; color: black;">
  <h1>Error compiling {{package_type}} package "{{package_name}}"</h1>
  <p><strong>Command:</strong></p>
  <pre style="white-space: pre-wrap;">{{command}}</pre>

pipeline/templatetags/pipeline.py

@@ -1,4 +1,5 @@
 import logging
+import re
 import subprocess
 
 from django import template
@@ -109,13 +110,19 @@ def render_compressed_sources(self, package, package_name, package_type):
         return method(package, paths, templates=templates)
 
     def render_error(self, package_type, package_name, e):
+        # Remove any ANSI escape sequences in the output.
+        error_output = re.sub(
+            re.compile(r"(?:\x1B[@-_]|[\x80-\x9F])[0-?]*[ -/]*[@-~]"),
+            "",
+            e.error_output)
+
         return render_to_string(
             "pipeline/compile_error.html",
             {
                 "package_type": package_type,
                 "package_name": package_name,
                 "command": subprocess.list2cmdline(e.command),
-                "errors": e.error_output,
+                "errors": error_output,
             },
         )