Readable error when toolchain paths not set.

[sarahmarshy]

Fixes #2360.

New error:
[Error] Toolchain path does not exist for IAR.
Current value: /default/path/that/doesnt/exist
(System exit before any build system calls)

[0xc0170]

LGTM

@theotherjimmy @screamerbg please review

[bridadan]

@sarahmarshy yay for better errors!

This problem affects build.py, test.py, and other script that actually uses the toolchains as well, not just make.py. Maybe there's a common place this can be implemented? My first thought was in settings.py, but that might not work correctly since it doesn't know what toolchain is actually being used.

This might need to go in tools/toolchains/__init__.py or something. Then it could error at runtime if it can't find it's own binary or something.

[sarahmarshy]

@bridadan I also thought of settings.py, but we don't want to error on a missing path for a tool not in use, as you said. I think the same problem might arise in tools/toolchain/__init__.py. It might be better suited in the constructor of each respective toolchain.

[sarahmarshy]

I would like to put it in the base class constructor, so we could have minimal changes, but the class names do not correspond 1:1 with the dictionary keys in TOOLCHAIN_PATHS. ARM_MICRO (class) uses ARM path to find system libraries. I will look into and submit a new PR.

[bridadan]

@sarahmarshy Yeah I think the constructor is probably the way to go, though @screamerbg may have a different idea.

[theotherjimmy]

I disagree with adding it to the toolchains' constructor for several reasons:

• It allows a constructor to fail
• Many uses of the toolchain classes do not invoke a compiler
  • the online export system, for example, would not be able to export to any non-armc5 export. That would be a BAD THING (TM)

Maybe we should check for the binaries at the moment we know that the mbedToolchain instance is going to need the executable. I'm thinking in the compile_sources method, the build_library method, and the link_program method. Personally (for reuse purposes) I would make it a decorator, and simply add the decorator to all the mentoined methods.

[sarahmarshy]

Moved to #2416

[bridadan]

@theotherjimmy Really good point, let's continue the discussion on #2416

[sarahmarshy]

@theotherjimmy one considetation: at the time of those functions, resource scan has already happened. It would just take some extra time to fail, but it is marginal.