scm: fix new command only use hg to clone code

[kevinong0108]

add check is hg file exist or not to avoid never try to use git.
But this patch maybe only for python 3.x

[theotherjimmy]

@kevinong0108 How is this any different from attempting to run hg and failing with almost the same error?

[kevinong0108]

@theotherjimmy it will break when hg clone mbed-os failed, so it never try to use git. (Windows environment)

[theotherjimmy]

This works, but only changes the clone method. Any other command on any other SCM can fail in the same way. Further, this makes the clone method aware that hg may not be installed. Instead, could we make all subprocess invocations fail with ProcessException when the command is not found or skip hg entirely (with a warning) when it's executable is not found?

[kevinong0108]

@theotherjimmy
If the repository is cloned, the mbed-cli will use correct tool. The problem happened on clone only for me.
And there is no rule for hg clone fail, so here use ProcessException to notify upper try and catch.

[theotherjimmy]

@kevinong0108 Mbed CLI is not the only way to clone a repo. Please update the PR with the proposed changes.

[bridadan]

This PR will need a rebase to bring in the fixes for the Circle CI configuration.