gm: report ExpectationsMismatch and RenderModeMismatch results separately

gm/tests/outputs/pipe-playback-failure/output-expected/stdout

Index: gm/tests/outputs/pipe-playback-failure/output-expected/stdout
===================================================================
--- gm/tests/outputs/pipe-playback-failure/output-expected/stdout	(revision 0)
+++ gm/tests/outputs/pipe-playback-failure/output-expected/stdout	(revision 0)
@@ -0,0 +1,12 @@
+GM: reading expectations from JSON summary file gm/tests/inputs/json/identical-pixels.json
+GM: drawing... selftest1 [300 200]
+GM: Ran 1 GMs
+GM: ... over  2 configs ["8888", "565"]
+GM: ...  and  5 modes   ["pipe", "replay", "rtree", "serialize", "tilegrid"]
+GM: ... so there should be a total of 7 tests.
+GM: Ran 7 tests: NoGpuContext=0 ImageMismatch=1 MissingExpectations=0 WritingReferenceImage=0
+GM: [*] 0 NoGpuContext:
+GM: [*] 1 ImageMismatch: comparison/selftest1-pipe
+GM: [ ] 0 MissingExpectations:
+GM: [*] 0 WritingReferenceImage:
+GM: (results marked with [*] will cause nonzero return value)