[csolution convert] Unsupported registered toolchain causes error in unrelated contexts

[brondani]

Having a toolchain registered without a compatible cmake file causes an error when running csolution convert even when such toolchain is not required:

$ export UNKNOWN_TOOLCHAIN_0_0_0=C:/tmp
$ csolution convert Hello.csolution.yml -S
error csolution: no toolchain cmake files found for 'UNKNOWN' in 'C:/cmsis-toolbox/latest/etc' directory

Expected behavior
The error is expected to be reported when calling csolution list toolchains but not when calling csolution convert if the unsupported toolchain is not required.

Environment (please complete the following information):

• csolution
• Version 2.7.0
• OS: [e.g. windows]