Improve error messages when VS cannot be found

The first experience for most developers who start working on ninja is
this cryptic error message:

bootstrapping ninja...
Traceback (most recent call last):
  File "configure.py", line 329, in <module>
    if platform.msvc_needs_fs():
  File "configure.py", line 89, in msvc_needs_fs
    stderr=subprocess.PIPE)
  File "python\bin\lib\subprocess.py", line 394, in __init__
    errread, errwrite)
  File "python\bin\lib\subprocess.py", line 644, in _execute_child
    startupinfo)
WindowsError: [Error 2] The system cannot find the file specified

This message happens when bootstrap.py first tries to invoke cl.exe and
it cannot be found. This change looks for cl.exe and warns if it is not
in the user's path, leading to this friendlier message:

bootstrapping ninja...
Traceback (most recent call last):
  File "configure.py", line 317, in <module>
    raise Exception('cl.exe not found. Run again from the Developer Command Prompt for VS')
Exception: cl.exe not found. Run again from the Developer Command Prompt for VS

0 files changed, 0 insertions, 0 deletions