ChatPromptBuilder raises the misleading ValueError when receiving incorect argument

[sebag90]

Describe the bug The ChatPromptBuilder raises a misleading ValueError when receiving incorrect argument. It complains that the prompt_source entry in not found, but it really needs the template argument

Error message ValueError: Input prompt_source not found in component prompt_builder.

Expected behavior ValueError: Input template not found in component prompt_builder.

Additional context I noticed this while migrating from the DynamicChatPromptBuilder and forgot to change prompt_source to ´template`

To Reproduce

from haystack.components.builders import ChatPromptBuilder
from haystack.components.generators.chat import OpenAIChatGenerator
from haystack.dataclasses import ChatMessage
from haystack import Pipeline
from haystack.utils import Secret

# no parameter init, we don't use any runtime template variables
prompt_builder = ChatPromptBuilder()
llm = OpenAIChatGenerator(api_key=Secret.from_token("<your-api-key>"), model="gpt-3.5-turbo")

pipe = Pipeline()
pipe.add_component("prompt_builder", prompt_builder)
pipe.add_component("llm", llm)
pipe.connect("prompt_builder.prompt", "llm.messages")

location = "Berlin"
language = "English"
system_message = ChatMessage.from_system("You are an assistant giving information to tourists in {{language}}")
messages = [system_message, ChatMessage.from_user("Tell me about {{location}}")]

res = pipe.run(data={"prompt_builder": {"template_variables": {"location": location, "language": language},
                                    "prompt_source": messages}})

print(res)

System:

• OS: mac
• GPU/CPU: M3 Pro
• Haystack version (commit or version number): 2.2.3