Change orElse and orNone to work on missing keys

[vlovgr]

• Add ConfigError#isMissingKey for checking whether an error occurred because the key was missing from the ConfigSource (for combined errors, whether all errors are due to missing keys).
• Change ConfigValue#orElse and orNone to only use the alternative value (or None) if the error is because of missing keys, as determined by ConfigError#isMissingKey.

For example, previously, a type conversion error for PORT in the following example:

> echo $PORT
changeme

scala> env[Int]("PORT").
     |   orElse(prop[Int]("port").
     |   orNone
res0: ConfigValue[Id, Option[Int]] = ConfigValue(Right(None))

would result in using the fallback values. This is probably not what you would expect, and this pull request changes orElse and orNone to only use fallback values if all previous errors are because of missing keys. In this example, we would stop at the environment variable PORT, because it was set but not of the expected type.

scala> env[Int]("PORT").
     |   orElse(prop[Int]("port").
     |   orNone
res0: ConfigValue[Id, Option[Int]] = ConfigValue(Left(WrongType(PORT, Environment, Right(changeme), changeme, Int, java.lang.NumberFormatException: For input string: "changeme")))

scala> res0.value.left.map(_.message).toString
res1: String = Left(Environment variable [PORT] with value [changeme] cannot be converted to type [Int]: java.lang.NumberFormatException: For input string: "changeme")

[codecov]

Codecov Report

Merging #124 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   95.34%   95.37%   +0.03%     
==========================================
  Files          43       43              
  Lines         687      692       +5     
  Branches        4        2       -2     
==========================================
+ Hits          655      660       +5     
  Misses         32       32

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

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