[Andrej730]

Hi! A small improvement for users building minisign from scratch - added SEND_ERROR error messages that make configuration stage fail, so it never get to generation stage where these errors get more obscure.

Before:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
/root/minisign/SODIUM_INCLUDE_DIR
   used as include directory in directory /root/minisign
SODIUM_LIBRARY
    linked by target "minisign" in directory /root/minisign

CMake Generate step failed.  Build files cannot be regenerated correctly.

After:

CMake Error at CMakeLists.txt:65 (message):
  Could not find libsodium headers (sodium.h).  Install libsodium or set
  SODIUM_INCLUDE_DIR.

CMake Error at CMakeLists.txt:68 (message):
  Could not find libsodium library.  Install libsodium or set SODIUM_LIBRARY.

[jedisct1]

Thank you!

I don't know much about cmake so I trust you :)