Better error handling when given a csproj or sln file

[svermeulen]

When I attempted to use this, I misread the command line instructions and passed in a .sln file or a .csproj file because this is how the dotnet formatter works. This resulted in this output:

+dotnet csharpier Foo.sln
/Foo.sln                                                                                                                 - failed to compile
total time:                                                                          243ms
Total files:                                                                           1
Failed syntax tree validation:                                                         0
Threw exceptions while formatting:                                                     0
files that threw exceptions while validating syntax tree:                              0

Which led me to believe that the sln file was not compiling, and not that I was supplying an invalid file. I understand now how it is meant to work, but wanted to just suggest that this error message be improved. When not given a .cs file or a directory, can it print out clearer instructions?

EDIT: Or, alternatively, actually process csproj / sln files and format all the C# scripts inside them

[belav]

I like the idea of allowing someone to pass a csproj/sln file, I created #399 for adding support for it.
Even with that implemented, if someone passes a path to csharpier that it can't format it should definitely let the user know.