Deprecate exception.message usages

In fb30d2d

     if isinstance(cause, UnreadableArtifact):
       return (target_address, cause.err.message)

However, `exception.message` isn't always a string, it's `args[0]` whatever is passed as first argument [1],
maybe string, maybe another wrapped exception. This results a later error when called by json.dump [2]

Further, `exception.message` is deprecated in python 3 [1], this PR drops such usages

[1] https://www.python.org/dev/peps/pep-0352
[2] bug stacktrace

  File "/Users/peiyu/.pex/install/pantsbuild.pants-0.0.0_fb30d2db66b95d2bb602cbaae60037c18d2710b6-py2-none-any.whl.5632d5cbfcdd1b48dcdcbd328644dfab1f356c58/pantsbuild.pants-0.0.0_fb30d2db66b95d2bb602cbaae60037c18d2710b6-py2-none-any.whl/pants/goal/run_tracker.py", line 263, in store_stats
    safe_file_dump(stats_file, json.dumps(stats))
...
Exception message: ConnectionError(ProtocolError('Connection aborted.', gaierror(8, 'nodename nor servnam

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/94495459

Bugs closed: 2656

Reviewed at https://rbcommons.com/s/twitter/r/3201/

src/python/pants/base/build_environment.py

@@ -33,7 +33,7 @@ def get_buildroot():
   try:
     return BuildRoot().path
   except BuildRoot.NotFoundError as e:
-    print(e.message, file=sys.stderr)
+    print(str(e), file=sys.stderr)
     sys.exit(1)
 
 

src/python/pants/goal/artifact_cache_stats.py

@@ -54,7 +54,7 @@ def format_vts(tgt, cause):
       """Format into (target, cause) tuple."""
       target_address = tgt.address.reference()
       if isinstance(cause, UnreadableArtifact):
-        return (target_address, cause.err.message)
+        return (target_address, str(cause.err))
       elif cause == False:
         return (target_address, 'uncached')
       else:

tests/python/pants_test/authentication/test_netrc_util.py

@@ -41,7 +41,7 @@ def test_netrc_parse_error(self, MockOsPath):
     with self.netrc('machine test') as netrc:
       with self.assertRaises(netrc.NetrcError) as exc:
         netrc._ensure_loaded()
-      assert re.search(r'Problem parsing', exc.exception.message)
+      assert re.search(r'Problem parsing', str(exc.exception))
 
   def test_netrc_no_usable_blocks(self, MockOsPath):
     with self.netrc('') as netrc:

tests/python/pants_test/goal/test_artifact_cache_stats.py

@@ -11,7 +11,7 @@
 import requests
 
 from pants.cache.artifact import ArtifactError
-from pants.cache.artifact_cache import UnreadableArtifact
+from pants.cache.artifact_cache import NonfatalArtifactCacheError, UnreadableArtifact
 from pants.goal.artifact_cache_stats import ArtifactCacheStats
 from pants.util.contextutil import temporary_dir
 from pants_test.base_test import BaseTest
@@ -21,7 +21,10 @@ class ArtifactCacheStatsTest(BaseTest):
   TEST_CACHE_NAME_1 = 'ZincCompile'
   TEST_CACHE_NAME_2 = 'Checkstyle_test_checkstyle'
   TEST_LOCAL_ERROR = UnreadableArtifact('foo', ArtifactError('CRC check failed'))
-  TEST_REMOTE_ERROR = UnreadableArtifact('bar', requests.exceptions.ConnectionError('Read time out'))
+  TEST_REMOTE_ERROR = UnreadableArtifact(
+    'bar',
+    NonfatalArtifactCacheError(requests.exceptions.ConnectionError('Read time out'))
+  )
   TEST_SPEC_A = 'src/scala/a'
   TEST_SPEC_B = 'src/scala/b'
   TEST_SPEC_C = 'src/java/c'
@@ -40,24 +43,24 @@ def test_add_hits(self):
         'num_hits': 0,
         'num_misses': 1,
         'hits': [],
-        'misses': [(self.TEST_SPEC_A, self.TEST_LOCAL_ERROR.err.message)]
+        'misses': [(self.TEST_SPEC_A, str(self.TEST_LOCAL_ERROR.err))]
       },
       {
         'cache_name': self.TEST_CACHE_NAME_1,
         'num_hits': 1,
         'num_misses': 1,
         'hits': [(self.TEST_SPEC_B, '')],
-        'misses': [(self.TEST_SPEC_C, self.TEST_REMOTE_ERROR.err.message)]
+        'misses': [(self.TEST_SPEC_C, str(self.TEST_REMOTE_ERROR.err))]
       },
     ]
 
     expected_hit_or_miss_files = {
       '{}.misses'.format(self.TEST_CACHE_NAME_2):
-        '{} {}\n'.format(self.TEST_SPEC_A, self.TEST_LOCAL_ERROR.err.message),
+        '{} {}\n'.format(self.TEST_SPEC_A, str(self.TEST_LOCAL_ERROR.err)),
       '{}.hits'.format(self.TEST_CACHE_NAME_1):
         '{}\n'.format(self.TEST_SPEC_B),
       '{}.misses'.format(self.TEST_CACHE_NAME_1):
-        '{} {}\n'.format(self.TEST_SPEC_C, self.TEST_REMOTE_ERROR.err.message),
+        '{} {}\n'.format(self.TEST_SPEC_C, str(self.TEST_REMOTE_ERROR.err)),
     }
 
     with self.mock_artifact_cache_stats(expected_stats,