[SPARK-15542][SPARKR] Make error message clear for script './R/install-dev.sh' when R is missing on Mac

https://issues.apache.org/jira/browse/SPARK-15542

## What changes were proposed in this pull request?

When running`./R/install-dev.sh` in **Mac OS EI Captain** environment, I got
```
mbp185-xr:spark xin$ ./R/install-dev.sh
usage: dirname path
```
This message is very confusing to me, and then I found R is not properly configured on my Mac when this script is using `$(which R)` to get R home.

I tried similar situation on CentOS with R missing, and it's giving me very clear error message while MacOS is not.
on CentOS:
```
[rootip-xxx-31-9-xx spark]# which R
/usr/bin/which: no R in (/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/lib/jvm/java-1.7.0-openjdk.x86_64/bin:/root/bin)
```
but on Mac, if not found then nothing returned and this is causing the confusing message for R build failure and running R/install-dev.sh:
```
mbp185-xr:spark xin$ which R
mbp185-xr:spark xin$
```

Here I just added a clear message for this miss configuration for R when running `R/install-dev.sh`.
```
mbp185-xr:spark xin$ ./R/install-dev.sh
Cannot find R home by running 'which R', please make sure R is properly installed.
```

## How was this patch tested?
Manually tested on local machine.

Author: Xin Ren <iamshrek@126.com>

Closes #13308 from keypointt/SPARK-15542.