Throw a custom exception when no match is found

Close #357

news/357.bugfix.md

@@ -0,0 +1 @@
+Throws an error with meaningful message when no candidate is found for one requirement.

pdm/exceptions.py

@@ -21,7 +21,7 @@ class CorruptedCacheError(PdmException):
     pass
 
 
-class PackageIndexError(PdmException):
+class CandidateNotFound(PdmException):
     pass
 
 

pdm/models/repositories.py

@@ -8,7 +8,7 @@
 
 from pdm import termui
 from pdm._types import CandidateInfo, Package, SearchResult, Source
-from pdm.exceptions import CandidateInfoNotFound, CorruptedCacheError
+from pdm.exceptions import CandidateInfoNotFound, CandidateNotFound, CorruptedCacheError
 from pdm.models.candidates import Candidate
 from pdm.models.requirements import (
     Requirement,
@@ -231,10 +231,16 @@ def dependency_generators(self) -> Iterable[Callable[[Candidate], CandidateInfo]
     def _find_candidates(self, requirement: Requirement) -> Iterable[Candidate]:
         sources = self.get_filtered_sources(requirement)
         with self.environment.get_finder(sources, True) as finder, allow_all_wheels():
-            return [
+            cans = [
                 Candidate.from_installation_candidate(c, requirement, self.environment)
                 for c in finder.find_all_candidates(requirement.project_name)
             ]
+        if not cans:
+            raise CandidateNotFound(
+                f"Unable to find candidates for {requirement.project_name}. There may "
+                "exist some issues with the package index or network condition."
+            )
+        return cans
 
     def search(self, query: str) -> SearchResult:
         pypi_simple = self.sources[0]["url"].rstrip("/")