[SPARK-26076][BUILD][MINOR] Revise ambiguous error message from load-spark-env.sh

## What changes were proposed in this pull request?

When I try to run scripts (e.g. `start-master.sh`/`start-history-server.sh ` in latest master, I got such error:
```
Presence of build for multiple Scala versions detected.
Either clean one of them or, export SPARK_SCALA_VERSION in spark-env.sh.
```

The error message is quite confusing. Without reading `load-spark-env.sh`,  I didn't know which directory to remove, or where to find and edit the `spark-evn.sh`.

This PR is to make the error message more clear. Also change the script for less maintenance when we add or drop Scala versions in the future.
As now with #22967, we can revise the error message as following(in my local setup):

```
Presence of build for multiple Scala versions detected (/Users/gengliangwang/IdeaProjects/spark/assembly/target/scala-2.12 and /Users/gengliangwang/IdeaProjects/spark/assembly/target/scala-2.11).
Remove one of them or, export SPARK_SCALA_VERSION=2.12 in /Users/gengliangwang/IdeaProjects/spark/conf/spark-env.sh.
Visit https://spark.apache.org/docs/latest/configuration.html#environment-variables for more details about setting environment variables in spark-env.sh.
```

## How was this patch tested?

Manual test

Closes #23049 from gengliangwang/reviseEnvScript.

Authored-by: Gengliang Wang <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: gengliangwang

bin/load-spark-env.sh

@@ -26,35 +26,40 @@ if [ -z "${SPARK_HOME}" ]; then
   source "$(dirname "$0")"/find-spark-home
 fi
 
+SPARK_ENV_SH="spark-env.sh"
 if [ -z "$SPARK_ENV_LOADED" ]; then
   export SPARK_ENV_LOADED=1
 
   export SPARK_CONF_DIR="${SPARK_CONF_DIR:-"${SPARK_HOME}"/conf}"
 
-  if [ -f "${SPARK_CONF_DIR}/spark-env.sh" ]; then
+  SPARK_ENV_SH="${SPARK_CONF_DIR}/${SPARK_ENV_SH}"
+  if [[ -f "${SPARK_ENV_SH}" ]]; then
     # Promote all variable declarations to environment (exported) variables
     set -a
-    . "${SPARK_CONF_DIR}/spark-env.sh"
+    . ${SPARK_ENV_SH}
     set +a
   fi
 fi
 
 # Setting SPARK_SCALA_VERSION if not already set.
 
 if [ -z "$SPARK_SCALA_VERSION" ]; then
+  SCALA_VERSION_1=2.12
+  SCALA_VERSION_2=2.11
 
-  ASSEMBLY_DIR2="${SPARK_HOME}/assembly/target/scala-2.11"
-  ASSEMBLY_DIR1="${SPARK_HOME}/assembly/target/scala-2.12"
-
-  if [[ -d "$ASSEMBLY_DIR2" && -d "$ASSEMBLY_DIR1" ]]; then
-    echo -e "Presence of build for multiple Scala versions detected." 1>&2
-    echo -e 'Either clean one of them or, export SPARK_SCALA_VERSION in spark-env.sh.' 1>&2
+  ASSEMBLY_DIR_1="${SPARK_HOME}/assembly/target/scala-${SCALA_VERSION_1}"
+  ASSEMBLY_DIR_2="${SPARK_HOME}/assembly/target/scala-${SCALA_VERSION_2}"
+  ENV_VARIABLE_DOC="https://spark.apache.org/docs/latest/configuration.html#environment-variables"
+  if [[ -d "$ASSEMBLY_DIR_1" && -d "$ASSEMBLY_DIR_2" ]]; then
+    echo "Presence of build for multiple Scala versions detected ($ASSEMBLY_DIR_1 and $ASSEMBLY_DIR_2)." 1>&2
+    echo "Remove one of them or, export SPARK_SCALA_VERSION=$SCALA_VERSION_1 in ${SPARK_ENV_SH}." 1>&2
+    echo "Visit ${ENV_VARIABLE_DOC} for more details about setting environment variables in spark-env.sh." 1>&2
     exit 1
   fi
 
-  if [ -d "$ASSEMBLY_DIR2" ]; then
-    export SPARK_SCALA_VERSION="2.11"
+  if [[ -d "$ASSEMBLY_DIR_1" ]]; then
+    export SPARK_SCALA_VERSION=${SCALA_VERSION_1}
   else
-    export SPARK_SCALA_VERSION="2.12"
+    export SPARK_SCALA_VERSION=${SCALA_VERSION_2}
   fi
 fi