Fix ConfigException#toString

[vlovgr]

Ensures that the exception message is included in toString, and therefore also in stack traces.

scala> loadConfig(env[Int]("A"), prop[Int]("B"))(_ + _).orThrow()
ciris.ConfigException: configuration loading failed with the following errors.

  - Environment variable [A] with value [x] cannot be converted to type [Int]: java.lang.NumberFormatException: For input string: "x".
  - Missing system property [B].

  at ciris.ConfigException$.apply(ConfigException.scala:46)
  at ciris.ConfigErrors$.toException$extension(ConfigErrors.scala:109)
  at ciris.syntax$EitherConfigErrorsSyntax$.$anonfun$orThrow$1(syntax.scala:22)
  at ciris.syntax$EitherConfigErrorsSyntax$.$anonfun$orThrow$1$adapted(syntax.scala:22)
  at scala.util.Either.fold(Either.scala:189)
  at ciris.syntax$EitherConfigErrorsSyntax$.orThrow$extension(syntax.scala:23)
  ... 36 elided

[codecov]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #68   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          28     28           
  Lines         433    433           
  Branches        3      3           
=====================================
  Hits          433    433

Impacted Files	Coverage Δ
.../shared/src/main/scala/ciris/ConfigException.scala	100% <100%> (ø)	⬆️

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