Ensure RxJava-related errors don't throw NPEs

[pettermahlen]

The getErrorHandler() method can return null; the safe way is to use the
onError(Throwable) hook.

[codecov]

Codecov Report

Merging #96 into master will not change coverage.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master      #96   +/-   ##
=========================================
  Coverage     78.12%   78.12%           
  Complexity      276      276           
=========================================
  Files            72       72           
  Lines          1431     1431           
  Branches         85       85           
=========================================
  Hits           1118     1118           
  Misses          297      297           
  Partials         16       16

Impacted Files	Coverage Δ	Complexity Δ
...src/main/java/com/spotify/mobius/rx2/RxMobius.java	68.42% <ø> (ø)	1 <0> (ø)	⬇️
...in/java/com/spotify/mobius/rx2/RxConnectables.java	56.81% <0%> (ø)	1 <0> (ø)	⬇️

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