Error out in case given file not found

[Dentrax]

Description

Currently, the following error means that checksums.txt does not found in the current directory, which is a bit confused message to see what's going on actually:

$ rekor-cli search --artifact checksums.txt --sha 852dc7cfc62513324debede3cdc7a159843b7a05ae0eb740d905b074e4798d21
Error: invalid argument "checksums.txt" for "--artifact" flag: Key: '' Error:Field validation for '' failed on the 'url' tag

We can revise the error to smth like: <FILE> does not found

$ rekor-cli version
GitVersion:    v0.3.0
GitCommit:     e4303a87b92081e162e080ff69f36830c1035c09
GitTreeState:  clean
BuildDate:     '2021-07-28T08:08:56Z'
GoVersion:     go1.16.6
Compiler:      gc
Platform:      darwin/amd64

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