1.x: replace non-serializable value of OnNextValue with its toString

[akarnokd]

When certain operators crash, they attach the current value to the exception via OnErrorThrowable.addValueAsLastCause, however, the value may not be serializable and the Throwable itself is, that causes a serialization exception.

This PR replaces such values with their String.valueOf representation or the crash message if toString failed on that object.

This was reported in #4562 with the suggestion of making the internal field transient. The problem with that is that upon deserialization, the field is null and there is no way of knowing if the original value was really null or just not serializable. Using String.valueOf at least gives a chance to know it was non-null and gets across some information about the original value.

[codecov-io]

Current coverage is 84.18% (diff: 50.00%)

Merging #4841 into 1.x will increase coverage by 0.12%

@@                1.x      #4841   diff @@
==========================================
  Files           287        287          
  Lines         17856      17868    +12   
  Methods           0          0          
  Messages          0          0          
  Branches       2704       2706     +2   
==========================================
+ Hits          15011      15043    +32   
+ Misses         1973       1961    -12   
+ Partials        872        864     -8   

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