Cryptic error message for invalid CONAN_CPU_COUNT values

[dstftw]

• I've read the CONTRIBUTING guide.
• I've specified the Conan version, operating system version and any tool that can be relevant.
• I've explained the steps to reproduce the error or the motivation/use case of the question/suggestion.

> conan --version
Conan version 1.11.2

Currently, when CONAN_CPU_COUNT env variable is set to an invalid non-integer value, a call to conan build for CMake based packages results in quite cryptic error message, e.g. here is an example for CONAN_CPU_COUNT=null:

ERROR: <snip>: Error in build() method, line 84
        cmake = CMake(self)
        ValueError: invalid literal for int() with base 10: 'null'

It may take some time to grasp what's the reason especially on large code bases or when environment is set by some intermediary like Jenkins' pipeline or so.
It would be nice to have a proper validation and more user friendly error message to cut down investigation time to a minimum.

[lasote]

Sure, thanks for open this.