[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 apache#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 apache#23049 from gengliangwang/reviseEnvScript.

Authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

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