Readable error when toolchain paths not set.

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)

tools/toolchains/__init__.py

@@ -188,6 +188,23 @@ LEGACY_TOOLCHAIN_NAMES = {
 }
 
 
+def check_toolchain_path(function):
+    """Check if the path to toolchain is valid. Exit if not.
+    Use this function as a decorator.  Causes a system exit if the path does
+    not exist. Execute the function as normal if the path does exist.
+
+    Positional arguments:
+    function -- the function to decorate
+    """
+    def perform_check(self, *args, **kwargs):
+        if not exists(self.toolchain_path):
+            print('[ERROR] Toolchain path does not exist for %s.\n'
+                  'Current value: %s' % (self.name, self.toolchain_path))
+            sys.exit()
+        return function(self, *args, **kwargs)
+    return perform_check
+
+
 class mbedToolchain:
     # Verbose logging
     VERBOSE = True
@@ -702,6 +719,7 @@ class mbedToolchain:
 
     # THIS METHOD IS BEING CALLED BY THE MBED ONLINE BUILD SYSTEM
     # ANY CHANGE OF PARAMETERS OR RETURN VALUES WILL BREAK COMPATIBILITY
+    @check_toolchain_path
     def compile_sources(self, resources, build_path, inc_dirs=None):
         # Web IDE progress bar for project build
         files_to_compile = resources.s_sources + resources.c_sources + resources.cpp_sources
@@ -900,6 +918,7 @@ class mbedToolchain:
             else:
                 raise ToolException(_stderr)
 
+    @check_toolchain_path
     def build_library(self, objects, dir, name):
         needed_update = False
         lib = self.STD_LIB_NAME % name
@@ -911,6 +930,7 @@ class mbedToolchain:
 
         return needed_update
 
+    @check_toolchain_path
     def link_program(self, r, tmp_path, name):
         needed_update = False
         ext = 'bin'

tools/toolchains/arm.py

@@ -81,6 +81,8 @@ class ARM(mbedToolchain):
         self.ar = join(ARM_BIN, "armar")
         self.elf2bin = join(ARM_BIN, "fromelf")
 
+        self.toolchain_path = TOOLCHAIN_PATHS['ARM']
+
     def parse_dependencies(self, dep_path):
         dependencies = []
         for line in open(dep_path).readlines():

tools/toolchains/gcc.py

@@ -110,6 +110,8 @@ class GCC(mbedToolchain):
         self.ar = join(tool_path, "arm-none-eabi-ar")
         self.elf2bin = join(tool_path, "arm-none-eabi-objcopy")
 
+        self.toolchain_path = tool_path
+
     def parse_dependencies(self, dep_path):
         dependencies = []
         buff = open(dep_path).readlines()

tools/toolchains/iar.py

@@ -101,6 +101,8 @@ class IAR(mbedToolchain):
         self.ar = join(IAR_BIN, "iarchive")
         self.elf2bin = join(IAR_BIN, "ielftool")
 
+        self.toolchain_path = TOOLCHAIN_PATHS['IAR']
+
     def parse_dependencies(self, dep_path):
         return [(self.CHROOT if self.CHROOT else '')+path.strip() for path in open(dep_path).readlines()
                 if (path and not path.isspace())]