Missing clear message for porter build when porter.yaml is missing

[Bharadwajshivam28]

Describe the bug

When running the command porter build if there is porter.yaml file missing then we not get a clear message

To Reproduce

Steps to reproduce the behavior:

1. mkdir test
2. cd test
3. porter create

Expected behavior

There should be a clear message when the file porter.yaml is missing current output is not clear

Porter Command and Output

[Bharadwajshivam28]

These are the lines where i found the issue is coming from-
1
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5

Do we really need 2 different lines of errors if the file is not there?
We can have a single line of error that will be unable to read manifest file : could not read the manifest at "": read /home/shivam/test: is a directory and we need to modify this error as it is not clear what the issue is..

I will be happy to raise a PR @schristoff