The `implicitNotFound` message for `Encoder` doesn't mention the name of
the type for which it can't find an encoder. Furthermore, it covers up
the fact that `Encoder` is the name of the relevant type class.
Hopefully this new message provides a little more specific type detail
while still giving the general message about which types are supported.
## What changes were proposed in this pull request?
Augment the existing message to mention that it's looking for an `Encoder` and what the type of the encoder is.
For example instead of:
```
Unable to find encoder for type stored in a Dataset. Primitive types (Int, String, etc) and Product types (case classes) are supported by importing spark.implicits._ Support for serializing other types will be added in future releases.
```
return this message:
```
Unable to find encoder for type Exception. An implicit Encoder[Exception] is needed to store Exception instances in a Dataset. Primitive types (Int, String, etc) and Product types (ca
se classes) are supported by importing spark.implicits._ Support for serializing other types will be added in future releases.
```
## How was this patch tested?
It was tested manually in the Scala REPL, since triggering this in a test would cause a compilation error.
```
scala> implicitly[Encoder[Exception]]
<console>:51: error: Unable to find encoder for type Exception. An implicit Encoder[Exception] is needed to store Exception instances in a Dataset. Primitive types (Int, String, etc) and Product types (ca
se classes) are supported by importing spark.implicits._ Support for serializing other types will be added in future releases.
implicitly[Encoder[Exception]]
^
```
Author: Cody Allen <ceedubs@gmail.com>
Closes#20869 from ceedubs/encoder-implicit-msg.
32acfa78c6