Common: fix flaky ThriftLoader test

[dilyand]

$e2 in ThriftLoaderSpec has erratic behaviour. It usually passes but sometimes can fail, like in:

[info] ThriftLoaderSpec
[info]   + toCollectorPayload should return a CollectorPayload for a valid Thrift CollectorPayload (even if parameterless)
[error]   x toCollectorPayload should return a Validation Failure for an invalid or corrupted Thrift CollectorPayload
[error]  Falsified after 79 passed tests.
[error]  > ARG_0: "/3"
[error]  > ARG_0_ORIGINAL: "㮘칂앛拪㲹芛먋礦/䭈㎵擧밯3摐㯡婓좁✣ퟖ뫝딨诩吞뒦쐊赩酼풎ￕ"
[error]  The seed is 3eIWhEidqcaUYtTXZXsFGlGEsZtRSNPtxhAu3jEpH6G=
[error]  
[error]  > Invalid(NonEmptyList(CPFormatViolation(Processor(ThriftLoaderSpec,v1),CPFormatViolation(2020-07-30T07:10:59.129Z,thrift,Fallback(error deserializing raw event: Cannot read. Remote side has closed. Tried to read 2 bytes, but only got 0 bytes. (This is often indicative of an internal error on the server side. Please check your server logs.))),RawPayload(/w==)))) is Invalid but Fallback(error deserializing raw event: Cannot read. Remote side has closed. Tried to read 2 bytes, but only got 0 bytes. (This is often indicative of an internal error on the server side. Please check your server logs.)) != Fallback(error deserializing raw event: Cannot read. Remote side has closed. Tried to read 1 bytes, but only got 0 bytes. (This is often indicative of an internal error on the server side. Please check your server logs.)) (ThriftLoaderSpec.scala:185)
[error] Actual:   ...read [2] byte...
[error] ...gs.))
[error] Expected: ...read [1] byte...
[error] ...gs.))
[info] Total for specification ThriftLoaderSpec
[info] Finished in 655 ms
[info] 2 examples, 1 failure, 0 error

[chuwy]

The message needs to assume "2 bytes" as well as "1 bytes"