Defer dtc parse errors in prevalidation

In the case that we can't prevalidate the dts because of a parse error, defer
dtc's error output until after attempting to build. If the Zephyr build also
fails, its error output is much more legible. If it doesn't, then fail with the
dtc output.

lambda/compiler.rb

@@ -35,7 +35,7 @@ def compile_board(board, keymap_data:, kconfig_data:, include_static_rhs: false)
       compile_command = ['compileZmk', '-b', board]
 
       if keymap_data
-        validate_devicetree!(keymap_data)
+        dts_parse_errors = validate_devicetree!(keymap_data)
         File.open('build.keymap', 'w') { |io| io.write(keymap_data) }
         compile_command << '-k' << './build.keymap'
       end
@@ -67,6 +67,13 @@ def compile_board(board, keymap_data:, kconfig_data:, include_static_rhs: false)
         raise CompileError.new('Compile failed to produce result binary', status: 500, log: compile_output)
       end
 
+      if dts_parse_errors
+        # DTS validation failed to parse the DTS, yet the Zephyr build
+        # nonetheless succeeded. We can't allow returning the result, since we
+        # were unable to check it for unsafe dts sections.
+        raise CompileError.new('Syntax error validating device-tree input', log: dts_parse_errors)
+      end
+
       result = File.read('zmk.uf2')
 
       [result, compile_output]
@@ -84,7 +91,11 @@ def validate_devicetree!(dtsi)
       Open3.capture3({}, 'dts2yml', unsetenv_others: true, stdin_data: dts)
 
     unless status.success?
-      raise CompileError.new('Syntax error checking device-tree input', log: stderr.split("\n"))
+      # The error output from dtc is much harder to understand than Zephyr's
+      # errors, and the line numbers don't match up due to preprocessing. Rather
+      # than raising these now, return the error output in order that it's only
+      # used in the case that the Zephyr build doesn't itself error.
+      return stderr.split("\n")
     end
 
     data =
@@ -101,6 +112,8 @@ def validate_devicetree!(dtsi)
       raise CompileError.new(
               "Device-tree included the non-permitted root sections: #{invalid_sections.inspect}", log: [])
     end
+
+    nil
   end
 
   # Lambda is single-process per container, and we get substantial speedups