title: How to change the message in a Python AssertionError?
body: <p>I'm writing per the following, in which I try to produce a decent error message when comparing two multiline blocks of Unicode text. The interior method that does the comparison raises an assertion, but the default explanation is useless to me</p>
  
  <p>I need to add something to code such as this below:</p>
  
  <pre><code>def assert_long_strings_equal(one, other):
      lines_one = one.splitlines()
      lines_other = other.splitlines()
      for line1, line2 in zip(lines_one, lines_other):
          try:
              my_assert_equal(line1, line2)
          except AssertionError, error:
              # Add some information to the printed result of error??!
              raise
  </code></pre>
  
  <p>I cannot figure out how to change the printed error message in the assertionerror I catch. I always get <code>AssertionError: u'something' != 'something else'</code>, which only shows the first line of the output.</p>
  
  <p><strong>How can I change the assertion message to print out whatever I want?</strong></p>
  
  <p>If it's relevant, I am using <code>nose</code> to run the test.</p>