run-tests: resume raising an exception when a server fails to start

Prior to 93228b2a1fc0, this exception was raised before the diff could be
printed.  By raising the exception after printing the diff, the location of the
failure can be identified, but it is also easier to locate test runs where this
occurs.  The test bot maintains a list of failed tests, separate from the wall
of diff output.

tests/run-tests.py

@@ -1762,10 +1762,6 b' class TestResult(unittest._TextTestResul'
             else:
                 servefail, lines = getdiff(expected, got,
                                            test.refpath, test.errpath)
-                if servefail:
-                    self.stream.write(
-                        'server failed to start (HGPORT=%s)' % test._startport)
-
                 self.stream.write('\n')
                 for line in lines:
                     line = highlightdiff(line, self.color)
@@ -1777,6 +1773,10 b' class TestResult(unittest._TextTestResul'
                         self.stream.write(line)
                         self.stream.flush()
 
+                if servefail:
+                    raise test.failureException(
+                        'server failed to start (HGPORT=%s)' % test._startport)
+
             # handle interactive prompt without releasing iolock
             if self._options.interactive:
                 if test.readrefout() != expected:

tests/test-run-tests.t

@@ -541,7 +541,7 b' Verify that when a process fails to star'
   >   $ echo 'abort: child process failed to start blah'
   > EOF
   $ rt test-serve-fail.t
-  server failed to start (HGPORT=*) (glob)
+  
   --- $TESTTMP/test-serve-fail.t
   +++ $TESTTMP/test-serve-fail.t.err
   @@ -1* +1,2 @@ (glob)
@@ -550,7 +550,7 b' Verify that when a process fails to star'
   
   ERROR: test-serve-fail.t output changed
   !
-  Failed test-serve-fail.t: output changed
+  Failed test-serve-fail.t: server failed to start (HGPORT=*) (glob)
   # Ran 1 tests, 0 skipped, 1 failed.
   python hash seed: * (glob)
   [1]