Improve missing submodule logic

[WardBrian]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

Implements the suggestions in #1216 to improve the error users see if they are missing the math submodule

Intended Effect:

The error goes from:

$ make examples/bernoulli/bernoulli

--- Compiling the main object file. This might take up to a minute. ---
g++    -c -o src/cmdstan/main.o src/cmdstan/main.cpp
src/cmdstan/main.cpp:1:10: fatal error: cmdstan/command.hpp: No such file or directory
    1 | #include <cmdstan/command.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [make/program:14: src/cmdstan/main.o] Error 1

to

$ make examples/bernoulli/bernoulli
ERROR: Missing Stan submodules.
We tried to find the Stan Math submodule at:
  stan/lib/stan_math/

The most likely source of the problem is CmdStan was cloned without
the --recursive flag.  To fix this, run the following command:
  git submodule update --init --recursive

And try building again
makefile:31: stan/lib/stan_math/make/libraries: No such file or directory
make: *** [makefile:361: stan/lib/stan_math/make/libraries] Error 1

How to Verify:

Check out without the submodules

Side Effects:

By making these hard-includes, some debugging gets a bit harder (e.g., the make print-* commands fail). I tried to mitigate this by including the MATH path in the error that gets shown

Documentation:

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)