Fair Scheduler Logging for the following cases can be useful for the user.
1. If **valid** `spark.scheduler.allocation.file` property is set, user can be informed and aware which scheduler file is processed when `SparkContext` initializes.
2. If **invalid** `spark.scheduler.allocation.file` property is set, currently, the following stacktrace is shown to user. In addition to this, more meaningful message can be shown to user by emphasizing the problem at building level of fair scheduler. Also other potential issues can be covered at this level as **Fair Scheduler can not be built. + exception stacktrace**
```
Exception in thread "main" java.io.FileNotFoundException: INVALID_FILE (No such file or directory)
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.<init>(FileInputStream.java:138)
at java.io.FileInputStream.<init>(FileInputStream.java:93)
at org.apache.spark.scheduler.FairSchedulableBuilder$$anonfun$buildPools$1.apply(SchedulableBuilder.scala:76)
at org.apache.spark.scheduler.FairSchedulableBuilder$$anonfun$buildPools$1.apply(SchedulableBuilder.scala:75)
```
3. If `spark.scheduler.allocation.file` property is not set and **default** fair scheduler file (**fairscheduler.xml**) is found in classpath, it will be loaded but currently, user is not informed for using default file so logging can be useful as **Fair Scheduler file: fairscheduler.xml is found successfully and will be parsed.**
4. If **spark.scheduler.allocation.file** property is not set and **default** fair scheduler file does not exist in classpath, currently, user is not informed so logging can be useful as **No Fair Scheduler file found.**
Also this PR is related with https://github.com/apache/spark/pull/15237 to emphasize fileName in warning logs when fair scheduler file has invalid minShare, weight or schedulingMode values.
## How was this patch tested?
Added new Unit Tests.
Author: erenavsarogullari <erenavsarogullari@gmail.com>
Closes#16813 from erenavsarogullari/SPARK-19466.
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