Tolerate unstarted builds in rebaseline server.
Tolerate unstarted builds in rebaseline server.

Currently server.py --rietveld-issue will crash if there are any
unstarted trybots on the latest patch. Instead of crashing, warn
that the bot hasn't started yet.

Review URL: https://codereview.chromium.org/758793003

rubenvb/skia,scroggo/skia,nfxosp/platform_external_skia,nvoron23/skia,UBERMALLOW/external_skia,AOSP-YU/platform_external_skia,PAC-ROM/android_external_skia,noselhq/skia,BrokenROM/external_skia,todotodoo/skia,VRToxin-AOSP/android_external_skia,AOSP-YU/platform_external_skia,OneRom/external_skia,shahrzadmn/skia,TeamTwisted/external_skia,timduru/platform-external-skia,MinimalOS-AOSP/platform_external_skia,Hikari-no-Tenshi/android_external_skia,nvoron23/skia,Igalia/skia,nfxosp/platform_external_skia,HalCanary/skia-hc,timduru/platform-external-skia,qrealka/skia-hc,tmpvar/skia.cc,UBERMALLOW/external_skia,Infinitive-OS/platform_external_skia,MarshedOut/android_external_skia,Jichao/skia,YUPlayGodDev/platform_external_skia,ominux/skia,tmpvar/skia.cc,Igalia/skia,Igalia/skia,MarshedOut/android_external_skia,Igalia/skia,OneRom/external_skia,DiamondLovesYou/skia-sys,google/skia,PAC-ROM/android_external_skia,TeamExodus/external_skia,w3nd1go/android_external_skia,nfxosp/platform_external_skia,vanish87/skia,MarshedOut/android_external_skia,samuelig/skia,AOSPB/external_skia,AOSP-YU/platform_external_skia,PAC-ROM/android_external_skia,todotodoo/skia,UBERMALLOW/external_skia,shahrzadmn/skia,amyvmiwei/skia,TeamTwisted/external_skia,timduru/platform-external-skia,invisiblek/android_external_skia,DiamondLovesYou/skia-sys,PAC-ROM/android_external_skia,boulzordev/android_external_skia,ominux/skia,YUPlayGodDev/platform_external_skia,aosp-mirror/platform_external_skia,MonkeyZZZZ/platform_external_skia,rubenvb/skia,ominux/skia,Jichao/skia,pcwalton/skia,AOSPB/external_skia,timduru/platform-external-skia,noselhq/skia,MarshedOut/android_external_skia,nvoron23/skia,MinimalOS-AOSP/platform_external_skia,amyvmiwei/skia,todotodoo/skia,pcwalton/skia,invisiblek/android_external_skia,PAC-ROM/android_external_skia,HalCanary/skia-hc,invisiblek/android_external_skia,spezi77/android_external_skia,nvoron23/skia,AOSP-YU/platform_external_skia,Jichao/skia,vanish87/skia,scroggo/skia,boulzordev/android_external_skia,todotodoo/skia,geekboxzone/mmallow_external_skia,aosp-mirror/platform_external_skia,YUPlayGodDev/platform_external_skia,google/skia,AOSPB/external_skia,w3nd1go/android_external_skia,google/skia,DiamondLovesYou/skia-sys,vanish87/skia,VRToxin-AOSP/android_external_skia,Infinitive-OS/platform_external_skia,boulzordev/android_external_skia,HalCanary/skia-hc,google/skia,geekboxzone/mmallow_external_skia,MonkeyZZZZ/platform_external_skia,BrokenROM/external_skia,Hikari-no-Tenshi/android_external_skia,nfxosp/platform_external_skia,TeamExodus/external_skia,boulzordev/android_external_skia,TeamTwisted/external_skia,aosp-mirror/platform_external_skia,todotodoo/skia,Igalia/skia,YUPlayGodDev/platform_external_skia,TeamTwisted/external_skia,qrealka/skia-hc,amyvmiwei/skia,rubenvb/skia,nfxosp/platform_external_skia,geekboxzone/mmallow_external_skia,vanish87/skia,google/skia,rubenvb/skia,google/skia,HalCanary/skia-hc,tmpvar/skia.cc,AOSPB/external_skia,MinimalOS-AOSP/platform_external_skia,spezi77/android_external_skia,rubenvb/skia,Jichao/skia,nfxosp/platform_external_skia,PAC-ROM/android_external_skia,AOSP-YU/platform_external_skia,VRToxin-AOSP/android_external_skia,scroggo/skia,tmpvar/skia.cc,MonkeyZZZZ/platform_external_skia,MonkeyZZZZ/platform_external_skia,noselhq/skia,todotodoo/skia,ominux/skia,MinimalOS-AOSP/platform_external_skia,pcwalton/skia,HalCanary/skia-hc,geekboxzone/mmallow_external_skia,AOSPB/external_skia,scroggo/skia,vanish87/skia,VRToxin-AOSP/android_external_skia,UBERMALLOW/external_skia,Igalia/skia,scroggo/skia,tmpvar/skia.cc,MonkeyZZZZ/platform_external_skia,jtg-gg/skia,shahrzadmn/skia,pcwalton/skia,jtg-gg/skia,w3nd1go/android_external_skia,samuelig/skia,Jichao/skia,invisiblek/android_external_skia,qrealka/skia-hc,w3nd1go/android_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,todotodoo/skia,Hikari-no-Tenshi/android_external_skia,scroggo/skia,TeamTwisted/external_skia,noselhq/skia,jtg-gg/skia,DiamondLovesYou/skia-sys,qrealka/skia-hc,vanish87/skia,HalCanary/skia-hc,shahrzadmn/skia,VRToxin-AOSP/android_external_skia,noselhq/skia,Hikari-no-Tenshi/android_external_skia,shahrzadmn/skia,w3nd1go/android_external_skia,TeamTwisted/external_skia,geekboxzone/mmallow_external_skia,w3nd1go/android_external_skia,HalCanary/skia-hc,shahrzadmn/skia,w3nd1go/android_external_skia,aosp-mirror/platform_external_skia,shahrzadmn/skia,pcwalton/skia,amyvmiwei/skia,Infinitive-OS/platform_external_skia,UBERMALLOW/external_skia,BrokenROM/external_skia,ominux/skia,nfxosp/platform_external_skia,pcwalton/skia,UBERMALLOW/external_skia,timduru/platform-external-skia,noselhq/skia,samuelig/skia,Infinitive-OS/platform_external_skia,AOSP-YU/platform_external_skia,aosp-mirror/platform_external_skia,VRToxin-AOSP/android_external_skia,HalCanary/skia-hc,TeamExodus/external_skia,noselhq/skia,MinimalOS-AOSP/platform_external_skia,rubenvb/skia,qrealka/skia-hc,TeamTwisted/external_skia,rubenvb/skia,Infinitive-OS/platform_external_skia,YUPlayGodDev/platform_e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