Offer suggestions on error

Currently Qbs throws unclear error messages when trying to build for the first time.

I've just manually set the toolchain and Qt profiles.

I ran qbs create-project on a test qmake project.

Trying to build is very unclear what the build system expects from the user:

$ qbs                                                                        
No build graph exists yet for this configuration.
Resolving project for configuration default
ERROR: /home/lilian/QtProjects/test/test.qbs:5:5 Dependency 'Qt.core' not found for product 'test'.

As a "new user"(I've used Qbs before but back then I also struggled with the profiles and the actual build):

• "build graph" does not explain what I did wrong(also, seems to throw the implementation detail as an error to the user).
• I didn't know that "default" has special meaning in that text(I realized that later when I tried the next command).
• The "ERROR" message is not helpful in explaining the actual cause of the issue.

Would be nice if in this case it would of suggested the command I could use to fix this issue. For example, how to set a default configuration.

This is mostly inspired by Rust error messages like this:

error[E0002]: non-exhaustive patterns: type std::option::Option<i32> is non-empty
  --> src/test/compile-fail/E0002.rs:14:5
   |
14 |     match x { } //~ ERROR E0002
   |     ^^^^^^^^^^^ not all cases are handled
   |
   = note: this may be fixed by possibly adding wildcards or more match arms.

Then I tried this:

$ qbs debug                                                                
No build graph exists yet for this configuration.
Resolving project for configuration debug
ERROR: /home/lilian/QtProjects/test/test.qbs:5:5 Dependency 'Qt.core' not found for product 'test'.

It should probably say that there is no "debug" configuration and list the available configurations, and maybe suggest how I could add one(doesn't need the full command, just a starting point).

I ran qbs config after that, hoping that I could find something in the help message that would indicate how I could set a configuration. Currently still lost(will sift again through the documentation of course).