Fix mvn detection

When mvn is not detected (not in executor's path), 'set -e' causes the
detection to terminate the script before the helpful error message can
be displayed.

Author: Matthew Farrellee <matt@redhat.com>

Closes #1181 from mattf/master-0 and squashes the following commits:

506549f [Matthew Farrellee] Fix mvn detection

make-distribution.sh

@@ -89,12 +89,12 @@ if [ -z "$JAVA_HOME" ]; then
   exit -1
 fi
 
-VERSION=$(mvn help:evaluate -Dexpression=project.version 2>/dev/null | grep -v "INFO" | tail -n 1)
-if [ $? != 0 ]; then
+if ! which mvn &>/dev/null; then
     echo -e "You need Maven installed to build Spark."
     echo -e "Download Maven from https://maven.apache.org/"
     exit -1;
 fi
+VERSION=$(mvn help:evaluate -Dexpression=project.version 2>/dev/null | grep -v "INFO" | tail -n 1)
 
 JAVA_CMD="$JAVA_HOME"/bin/java
 JAVA_VERSION=$("$JAVA_CMD" -version 2>&1)